musb INDEX register is getting modified/corrupted during temporary
un-locking in a SMP system. Set this register with proper value
after re-acquiring the lock

Scenario:
---------
CPU1 is handling a data transfer completion interrupt received for
the CLASS1 EP
CPU2 is handling a CLASS2 thread which is queuing data to musb for
transfer

Below is the error sequence:

         CPU1                   |             CPU2
--------------------------------------------------------------------
Data transfer completion inter- |
rupt recieved.                  |
                                |
musb INDEX reg set to CLASS1 EP |
                                |
musb LOCK is acquired.          |
                                |
                                | CLASS2 thread queues data.
                                |
                                | CLASS2 thread tries to acquire musb
                                | LOCK but lock is already taken by
                                | CLASS1, so CLASS2 thread is
                                | spinning.
                                |
From Interrupt Context musb     |
giveback function is called     |
                                |
The giveback function releases  | CLASS2 thread now acquires LOCK
LOCK                            |
                                |
ClASS1 Request's completion cal-| ClASS2 schedules the data transfer and
lback is called                 | sets the MUSB INDEX to Class2 EP number
                                |
Interrupt handler for CLASS1 EP |
tries to acquire LOCK and is    |
spinning                        |
                                |
Interrupt for Class1 EP acquires| Class2 completes the scheduling etc and
the MUSB LOCK                   | releases the musb LOCK
                                |
Interrupt for Class1 EP schedul-|
es the next data transfer       |
but musb INDEX register is still|
set to CLASS2 EP                |

Since the MUSB INDEX register is set to a different endpoint, we
read and modify the wrong registers. Hence data transfer will not
happen properly. This results in unpredictable behavior

So, the MUSB INDEX register is set to proper value again when
interrupt re-acquires the lock

Cc: stable@vger.kernel.org
Signed-off-by: Supriya Karanth <supriya.karanth@stericsson.com>
Signed-off-by: Praveena Nadahally <praveen.nadahally@stericsson.com>
Reviewed-by: srinidhi kasagar <srinidhi.kasagar@stericsson.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>