serial: imx: Rename register fields to match newer reference manuals

Only the reference manual for the i.MX1 (I have MC9328MX1RM/D Rev 5 from
2004) uses TDMAEN and RDMAEN for these. All reference manuals for the
newer chips use TXDMAEN and RXDMAEN. Update to the newer name with the
assumption that most imx users don't use an imx1 any more.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>