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"