serial: fsl_lpuart: restore UARTCR2 after watermark setup is done

serial: fsl_lpuart: restore UARTCR2 after watermark setup is done

Function lpuart_setup_watermark() clears some bits in register UARTCR2
before writing FIFO configuration registers as required by hardware.
But it should restore UARTCR2 after that.  Otherwise, we end up changing
UARTCR2 register when setting up watermark, and that is not really
desirable.  At least, when low-level debug and earlyprint is enabled,
serial console is broken due to it.

Fix the problem by restoring UARTCR2 register at the end of function
lpuart_setup_watermark().

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>