qtnfmac: fix bg_scan_period parameter processing

Do not process bg_scan_period parameter in qtnfmac driver.
Pass correct values as is. In the case of invalid values
pass default value. Leave further processing to firmware.

Signed-off-by: Andrey Shevchenko <ashevchenko@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>