at76c50x-usb: convert at76_debug to an unsigned int

at76_debug should be an unsigned int as it used as a bit field.  In
fact, modprobe fails when trying to set at76_debug's high bit.

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