mxser: rename mxser_board::chip_flag to must_hwid

It is named IsMoxaMustChipFlag in the original Moxa's driver. Over the
time, it was reduced to chip_flag which doesn't make much sense. So dub
it again by what it is: must_hwid (mxser_must_get_hwid assigns to it).

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-8-jslaby@suse.cz


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