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

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

[ Upstream commit 5593a37379f07f4f01661c22eed26852dd0dfc83 ]

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: 109a951a9f1f ("serial: tegra: Read DMA status before terminating")
Signed-off-by: Sasha Levin <sashal@kernel.org>