serial: imx: remove null check of sport in suspend/resume function

platform_get_drvdata always retrun a valid value after probe succeed.

It also fixed smatch warnings:
drivers/tty/serial/imx.c:1376 serial_imx_suspend() warn: variable dereferenced before check 'sport' (see line 1372)
drivers/tty/serial/imx.c:1392 serial_imx_resume() warn: variable dereferenced before check 'sport' (see line 1388)

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>