Use explicit protocol discriminator for NC SS/USSD session

In gsm0911_rcv_nc_ss() we sometimes use pdisc parsed from msgb and
sometimes constant. This function is only called when protocol
discriminator is GSM48_PDISC_NC_SS so there's no point in parsing it
again from msgb.

Let's make it consistent and always use constant.

Change-Id: Iae40bf9906fe676ff817c709120015fca4c9e042
1 file changed