brcmfmac: use kcalloc() to prevent integer overflow

The multiplication here looks like it could overflow.  I've changed it
to use kcalloc() to prevent that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>