linux-wandboard

MLK-14980 tty: serial: fsl_lpuart: remove unnecessary .async_tx_ack()

lpuart only use NXP/FSL eDMA dmaengine in i.MX/Vybrid/LS1021a platform,
and eDMA driver don't reuse descriptor then no need to check the
flag DMA_CTRL_ACK. And current eDMA driver use virt chan mechanism and
free tx_descriptor memory after .callback(), but .lpuart_timer_func()
first to terminate the chans that free the tx_descriptor memory, then
access the tx->flags, which cause kmem_cache_alloc() failed to allocate
the freed memory. So remove the unnecessary .async_tx_ack().

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>