p54: Move TX/RX code

Copy the TX/RX code from p54common.c into a new file txrx.c
Signed-off-by: NChristian Lamparter <chunkeey@web.de>
Signed-off-by: NLarry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>