isdn: mISDN: Fix potential NULL pointer dereference of kzalloc

Allocating memory via kzalloc for phi may fail and causes a
NULL pointer dereference. This patch avoids such a scenario.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>