drivers: isdn: isdnloop: isdnloop.c: remove assignment of variables in if...

drivers: isdn: isdnloop: isdnloop.c: remove assignment of variables in if conditions, in accordance with the CodingStyle.

Signed-off-by: Bas Peters <baspeters93@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>