serial: linflexuart: correct dma operations to fix Sleep-To-RAM failure

The commit a7823395 ("tty: serial: linflexuart: UART support for FIFO
with DMA") put the dma operations in wrong place, causing Sleep-To-RAM
failure from resume, so correct the location of these operations to fix
the issue.

Signed-off-by: Zhantao Tang <zhantao.tang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>