serial: imx: TX DMA: clean up sg initialization

Inverting the logic of the if statement for the sg initialization
makes the if statement easier and better to read.

No functional change.

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