staging: wilc1000: fix NULL comparison style

This patch fixes NULL comparison style found by checkpatch.pl.

CHECK: Comparison to NULL could be written "!net"
CHECK: Comparison to NULL could be written "!net->ieee80211_ptr"
CHECK: Comparison to NULL could be written "!net->ieee80211_ptr->wiphy"

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>