brcmfmac: Using zero instead of NULL

Sparse complains that we use zero instead of NULL here.  In fact, the
initialization is wrong and should be removed.  Doing these kinds of
bogus initializations means that GCC can't detect unitialized variables
and leads to bugs.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>