LinuxKPI: 802.11: Stop checking for failures from malloc(M_WAITOK)

As a consequence lkpi_ieee80211_ifalloc() now does not fail. Remove
unneeded NULL check.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45852

(cherry picked from commit 7bcb1228558b4325fab39616e2e3b2573a9e7da6)