staging: wilc1000: fix NULL comparison style

This patch changes NULL comparison style to use ! operator found by
checkpatch.pl

CHECK: Comparison to NULL could be written "!pstrWFIDrv"
drivers/staging/wilc1000/host_interface.c:5324:

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