mxser: extract port ISR

The ISR is terrible mix of letters. Let's extract the proper per-port
handling to a separate function called mxser_port_isr. This way, we can
actually see what both mxser_interrupt and mxser_port_isr do now.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-14-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>