brcmfmac: only lock and unlock fws if fws is not null

There is a null ptr check for fws to set bcmc_credit_check, however,
there a lock and unlock on fws should only performed if fwts is
also not null to also avoid a potential null pointer deference.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>