Bluetooth: btusb: Fix isochronous interface assignments

The recent MacBook's with multi-function USB interfaces for HID and
Bluetooth operation have the isochronous interface on number 3 instead
of number 1. Store the interface number and use it.

P:  Vendor=05ac ProdID=8290 Rev= 1.40
S:  Manufacturer=Broadcom Corp.
S:  Product=Bluetooth USB Host Controller
C:* #Ifs= 6 Cfg#= 1 Atr=e0 MxPwr=  0mA
A:  FirstIf#= 2 IfCount= 4 Cls=ff(vend.) Sub=01 Prot=01
I:* If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=01 Driver=usbhid
E:  Ad=85(I) Atr=03(Int.) MxPS=   8 Ivl=10ms
I:* If#= 1 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=02 Driver=usbhid
E:  Ad=86(I) Atr=03(Int.) MxPS=   8 Ivl=10ms
I:* If#= 2 Alt= 0 #EPs= 3 Cls=ff(vend.) Sub=01 Prot=01 Driver=btusb
E:  Ad=81(I) Atr=03(Int.) MxPS=  16 Ivl=1ms
E:  Ad=82(I) Atr=02(Bulk) MxPS=  64 Ivl=0ms
E:  Ad=02(O) Atr=02(Bulk) MxPS=  64 Ivl=0ms
I:* If#= 3 Alt= 0 #EPs= 2 Cls=e0(wlcon) Sub=01 Prot=01 Driver=btusb
E:  Ad=83(I) Atr=01(Isoc) MxPS=   0 Ivl=1ms
E:  Ad=03(O) Atr=01(Isoc)...