rt2x00usb: Fix a warning message in 'rt2x00usb_watchdog_tx_dma()'

'forced' is duplicated in the message, axe one of them.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>