serial: imx: unmap sg in case of dmaengine_prep_slave_sg() failure

In case dmaengine_prep_slave_sg() fails, add the missing dma_unmap_sg().

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Anton Bondarenko <anton_bondarenko@mentor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>