brcm80211: fmac: use spinlock calls saving irq flags in brcmf_enq_event()

This function is executed within irq context. The call spin_unlock_irq
does enable interrupts which is not desired in the irq context. This patch
replaces them using the spin_loc_irqsave and spin_unlock_irqrestore
functions.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Kan Yan <kanyan@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>