[ARM] 4994/1: <IMX UART>: Move error handling into execution path

Move the error handling code for erroneous receive characters into
execution path. This makes the code more readable and the compiler
should know how to optimize this, right?

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