mxser: use tty_port_tty guard() in mxser_port_isr()

mxser: use tty_port_tty guard() in mxser_port_isr()

Use scoped_guard() and reorder the function. This is done separately
from the other changes, as it is slighly more intrusive: scoped_guard()
handles the have-tty case and returns. The non-tty case is done at the
end of the function.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250814072456.182853-7-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>