staging: wilc100: remove WILC_NULL usage

Use the "real" NULL value, don't try to be cute and define your own
value for something that the compiler obviously supports.

Cc: Johnny Kim <johnny.kim@atmel.com>
Cc: Rachel Kim <rachel.kim@atmel.com>
Cc: Dean Lee <dean.lee@atmel.com>
Cc: Chris Park <chris.park@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>