tty: serial: fsl_lpuart: Check the return value of dmaengine_tx_status

Coverity reports the Unchecked return value (CHECKED_RETURN) warning:
Calling dmaengine_tx_status without checking return value.

So here add the return value check for dmaengine_tx_status() function to
make coverity happy.

Fixes: cf9aa72d

 ("tty: serial: fsl_lpuart: optimize the timer based EOP logic")
Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20230522025111.3747-1-sherry.sun@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>