pch_uart: check kzalloc result in dma_handle_tx()

Reported by coccinelle:

drivers/tty/serial/pch_uart.c:979:1-14: alloc with no test, possible model on line 994

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>