staging: wilc1000: fixes comparison to NULL could be written

This patch fixes the checks reported by checkpatch.pl
for comparison to NULL could be written.
Signed-off-by: NLeo Kim <leo.kim@atmel.com>
Signed-off-by: NTony Cho <tony.cho@atmel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>