staging: wilc1000: replace MALLOC_WILC_BUFFER() macro to avoid possible memory leak

The MALLOC_WILC_BUFFER() macro was using a return statement, and didn't
take care of possible memory leaks and subsequent bugs when it was failing
after succeeding some allocations. This patch corrects this behavior.

Signed-off-by: Raphaël Beamonte <raphael.beamonte@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>