usb: musb: clear AUTOSET while clearing DMAENAB

On the completion of tx dma, dma is disabled by clearing MUSB_TXCSR_DMAENAB in
TXCSR. If MUSB_TXCSR_AUTOSET was set in txstate() it will remain set although
it is not needed in PIO mode. Clear it as soon as it is not needed.

Signed-off-by: Mian Yousaf Kaukab <mian.yousaf.kaukab@stericsson.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>