quoted

It is not easy to propagate that return code to the callers in
uart_configure_port/uart_suspend_port/uart_resume_port, unless
we change all UART drivers, and it is unclear what those would
do with the return code.

Instead, this patch uses a runtime warning to replace the
compiletime warning. I have checked that the regulator in question
is hardcoded to a fixed-voltage GPIO regulator, and that should
never fail to get enabled if I understand the code right.  

Looks OK to me:

If it was a concern for a real driver I think I'd perform a hangup on the
port at that moment. That's what happens if for example you hot unplug a
serial port which would be approximately the same as finding out you
can't power it back up.

Alan