tty: serial: 8250: Fix wording in runtime-PM comments

Frans reworded the two comments with better English for better
understanding. His review hit the mailing list after the patch got
applied so here is an incremental update.

Reported-by: Frans Klaver <frans.klaver@xsens.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>