at76c50x-usb: Supply additional parameters to at76_start_monitor scan request

For my Linksys WUSB11 at76c503-i3861 device, scanning fails without
probe_delay, min_channel_time, and max_channel_time specified for the
scan request.  These values were found by checking scan requests from
the at76_usb driver.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>