mxser: introduce mxser_16550A_or_MUST helper

This check is used in the code on many places. Either in positive or
negative form. So introduce a helper called mxser_16550A_or_MUST for
this purpose. And replace all the occurrences.

This simplifies many ifs in there.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-67-jslaby@suse.cz


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