USB: option: fix Cinterion AHxx enumeration

In certain kernel configurations where the cdc_ether and option drivers
are compiled as modules there can occur a race condition in enumeration.
This causes the option driver to enumerate the ethernet(wwan) interface
as usb-serial interfaces.

usb-devices output for the modem:
T:  Bus=01 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  5 Spd=480 MxCh= 0
D:  Ver= 2.00 Cls=ef(misc ) Sub=02 Prot=01 MxPS=64 #Cfgs=  1
P:  Vendor=1e2d ProdID=0055 Rev=00.00
S:  Manufacturer=Cinterion
S:  Product=AHx
C:  #Ifs= 6 Cfg#= 1 Atr=e0 MxPwr=10mA
I:  If#= 0 Alt= 0 #EPs= 2 Cls=ff(vend.) Sub=ff Prot=ff Driver=option
I:  If#= 1 Alt= 0 #EPs= 2 Cls=ff(vend.) Sub=ff Prot=ff Driver=option
I:  If#= 2 Alt= 0 #EPs= 2 Cls=ff(vend.) Sub=ff Prot=ff Driver=option
I:  If#= 3 Alt= 0 #EPs= 3 Cls=ff(vend.) Sub=ff Prot=ff Driver=option
I:  If#= 4 Alt= 0 #EPs= 1 Cls=02(commc) Sub=06 Prot=00 Driver=cdc_ether
I:  If#= 5 Alt= 1 #EPs= 2 Cls=0a(data ) Sub=00 Prot=00 Driver=cdc_ether

Signed-off-by: John Ernberg <john.ernberg@actia.se>
Fixes: 1941138e ("USB: added support for Cinterion's products...")
Cc: stable <stable@vger.kernel.org>	# v3.9: 8ff10bdb
Signed-off-by: Johan Hovold <johan@kernel.org>