When unbinding the console uart, we want to power down the uart hardware
on remove. For the console uart, the normal shutdown path will never get
called as the cons_filp stays open. Let's rearrange the dma related
functions a bit so we can call driver shutdown also on console uart rebind.

Currently we set up->dma on probe, but that causes issues calling
omap_8250_shutdown() on remove. The dma startup will not get called on
the next rebind as we still have up->dma set from probe.

Note that serial8250_release_dma() already checks for dma so we can
remove the check for it in 8205_omap driver.

With these changes we also avoid hogging dma virtual channels for the
unused uarts that may be limited on some devices.
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20230508082014.23083-5-tony@atomide.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>