staging: wilc1000: fix NULL comparison in host_int_remove_wep_key

This patch fixes NULL comparison style in host_int_remove_wep_key found
by checkpatch.

CHECK: Comparison to NULL could be written "!wfi_drv"
FILE: drivers/staging/wilc1000/host_interface.c:4300

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