Enabling the core interrupts in complete is too late for
XHCI, and stops it from proper operation. The root of the
problem is due to a disagreement between dwc3-omap and XHCI
about when IRQs should be enabled.

As it turns out, ->resume's documentation states that:

"... generally the driver is expected to start working
again, responding to hardware events and software requests
(the device itself may be left in a low-power state, waiting
for a runtime resume to occur) ..."

From that we infer that IRQs must be unmasked by the end of
->resume().

Due to that, we will remove ->prepare() and ->complete() and
disable/enable interrupts in ->suspend()/->resume().
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: George Cherian <george.cherian@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>