serial: core: Fix regression when runtime PM is not enabled

Commit 45a3a8ef ("serial: core: Revert checks for tx runtime PM state")
caused a regression for Sun Ultra 60 for the sunsab driver as reported by
Nick Bowler <nbowler@draconx.ca>.

We need to add back the check runtime PM enabled state for serial port
controller device, I wrongly assumed earlier we could just remove it.

Fixes: 45a3a8ef

 ("serial: core: Revert checks for tx runtime PM state")
Cc: stable <stable@kernel.org>
Reported-by: Nick Bowler <nbowler@draconx.ca>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20240325071649.27040-1-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>