serial: fsl_lpuart: Remove unneeded check for 'res'

'res' will be automatically checked inside devm_ioremap_resource(), so there is
no need to explicitly perform a NULL check.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Jingchang Lu <jingchang.lu@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>