brcmfmac: Set timeout value when configuring power save

Set the timeout value as per cfg80211's set_power_mgmt() request. If the
requested value value is left undefined we set it to 2 seconds, the
maximum supported value.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200721112302.22718-1-nsaenzjulienne@suse.de