staging: wilc1000: replace explicit NULL comparisons with !

This patch replace explicit NULL comparison with ! or unmark
operator to simplify code.
Reported by checkpatch.pl for comparison to NULL could be written "!XXX" or "XXX".

Signed-off-by: Leo Kim <leo.kim@atmel.com>
Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>