mxser: introduce enum mxser_must_hwid

Provided the MUST HW ID is an enumeration, define one (enum
mxser_must_hwid) and use it in the code.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-9-jslaby@suse.cz


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