brcmfmac: don't WARN when there are no requests

When n_reqs == 0 there is nothing to do so it doesn't make sense to
search for requests and issue a warning because none is found.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>