serial_mxc: Fix setup of UARTx_UFCR register

This patch writes the default values for TXTL and RXTL to UARTx_UFCR.
Without this patch some older kernel versions crash as UARTx_UFCR was
not always correctly initialized.

Signed-off-by: Maximilian Schwerin <maximilian.schwerin@tigris.de>