brcmfmac: use irq safe spinlock in brcmf_sdbrcm_txdata()

Firmware-signalling needs transmit to firmware to be atomic and
uses a spinlock with irq disabled. Therefor, brcmf_sdbrcm_txdata()
should not use spin_unlock_bh() as it would enable the interrupts.

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