tty: serial: tegra: Activate RX DMA transfer by request

[ Upstream commit d5e3fadb ]

This allows DMA engine to go into runtime-suspended mode whenever there is
no data to receive, instead of keeping DMA active all the time while TTY
is opened (i.e. permanently active in practice, like in the case of UART
Bluetooth).

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20200112180919.5194-2-digetx@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: 109a951a ("serial: tegra: Read DMA status before terminating")
Signed-off-by: Sasha Levin <sashal@kernel.org>