staging: wfx: fix value of scan timeout

Before to start the scan request, the firmware signals (with a null
frame) to the AP it won't be able to receive data. This frame can be
long to send: up to 512TU. The current calculus of the scan timeout does
not take into account this delay.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200515083325.378539-5-Jerome.Pouiller@silabs.com


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