brcmfmac: allow device tree node without 'interrupts' property

As described in the device tree bindings for 'brcm,bcm4329-fmac'
nodes, the interrupts property is optional. So adding a check
for the presence of this property before attempting to parse
and map the interrupt. If not present or parsing fails return
and fallback to in-band sdio interrupt.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>