Adding support for Quectel EG91 LTE module.

The interface layout is same as for EG95.

usb-devices output:
T:  Bus=01 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#=  3 Spd=480 MxCh= 0
D:  Ver= 2.00 Cls=ef(misc ) Sub=02 Prot=01 MxPS=64 #Cfgs=  1
P:  Vendor=2c7c ProdID=0191 Rev=03.18
S:  Manufacturer=Android
S:  Product=Android
C:  #Ifs= 5 Cfg#= 1 Atr=a0 MxPwr=500mA
I:  If#=0x0 Alt= 0 #EPs= 2 Cls=ff(vend.) Sub=ff Prot=ff Driver=(none)
I:  If#=0x1 Alt= 0 #EPs= 3 Cls=ff(vend.) Sub=00 Prot=00 Driver=(none)
I:  If#=0x2 Alt= 0 #EPs= 3 Cls=ff(vend.) Sub=00 Prot=00 Driver=(none)
I:  If#=0x3 Alt= 0 #EPs= 3 Cls=ff(vend.) Sub=00 Prot=00 Driver=(none)
I:  If#=0x4 Alt= 0 #EPs= 3 Cls=ff(vend.) Sub=ff Prot=ff Driver=qmi_wwan

Interfaces:

0: Diag
1: GNSS
2: AT-command interface/modem
3: Modem
4: QMI
Signed-off-by: Tomaz Solc <tomaz.solc@tablix.org>
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>