brcm80211: fmac: change allocation flag in brcmf_enq_event() function

As the function is called from atomic context it should not do the
kzalloc call with GFP_KERNEL.

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