brcmfmac: remove comparison to bool in brcmf_fws_attach()

Fix the following coccicheck warning:

drivers/net/wireless/broadcom/brcm80211/brcmfmac/fwsignal.c:2359:6-40:
WARNING: Comparison to bool

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Chi-hsien Lin <chi-hsien.lin@cypress.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200426094053.23132-1-yanaijie@huawei.com