brcmsmac: remove Comparison to bool in brcms_b_txstatus()

Fix the following coccicheck warning:

drivers/net/wireless/broadcom/brcm80211/brcmsmac/main.c:1060:6-12:
WARNING: Comparison to bool

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200504113357.41422-1-yanaijie@huawei.com