[ARM] 4995/1: <IMX UART>: Do not use URXD_CHARRDY for polling

Do not use the URXD_CHARRDY bit for polling for new characters. This works
on i.MX1, but on MX31 the datasheet states that this bit should not be
used for polling. On MX27 it is even worse, here we get a bus error when
we access the read FIFO when no character is present.
Instead, use USR2_RDR (receive data ready) bit.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>