rt2800usb: remove watchdog

On rt2800usb, if we do not get TX status from HW, we assume frames were
posted and after entry->last_action timeout, we forcibly provide TX
status to mac80211. So it's not possible to detect hardware TX hung
based on the timeout. Additionally TXRQ_PCNT tells on number of frames
in the Packet Buffer (buffer between bus interface and chip MAC
subsystem), which can be non zero on normal conditions. To check HW hung
we will need provide some different mechanism, for now remove watchdog
as current implementation is wrong and not useful.

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