staging: wilc1000: linux_mon: fix NULL comparison style

This patch fixes NULL comparsion style found by checkpatch.pl

CHECK: Comparison to NULL could be written "!wilc_wfi_mon"
CHECK: Comparison to NULL could be written "!skb"
CHECK: Comparison to NULL could be written "!skb"
CHECK: Comparison to NULL could be written "!dev"
CHECK: Comparison to NULL could be written "!mgmt_tx"
CHECK: Comparison to NULL could be written "!mgmt_tx->buff"
CHECK: Comparison to NULL could be written "!wilc_wfi_mon"
CHECK: Comparison to NULL could be written "!mon_priv"
CHECK: Comparison to NULL could be written "!priv"
CHECK: Comparison to NULL could be written "wilc_wfi_mon"

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