staging: wfx: check value of beacon_int

Firmware dislike when beacon_int value is 0. This patch add some
warnings in case it would happen.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200410133239.438347-18-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>