mxser: cleanup mxser_dtr_rts

mxser_dtr_rts now does all inb, outb, & or | in a single statement.  The
code is hard to follow, so make it more readable by doing one-line =
one-statement.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-57-jslaby@suse.cz


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