rt2800: do not enable watchdog by default

Make watchdog disabled by default and add module parameter to enable it.

User will have to create file in /etc/modprobe.d/ with

options rt2800lib watchdog=1

to enable the watchdog or load "rt2800lib watchdog=1" module manually
before loading rt2800{soc,pci,usb} module.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>