p54: Move TX/RX code

Copy the TX/RX code from p54common.c into a new file txrx.c

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>