brcmfmac: fix error handling of irq_of_parse_and_map

Return value of irq_of_parse_and_map() is unsigned int, with 0
indicating failure, so testing for negative result never works.

Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Cc: stable@vger.kernel.org # v3.17
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>