mxser: dedup mxser_must_set_enhance_mode

There are multiple copies of setting the EFR register. Separate it to a
new universal function called __mxser_must_set_EFR. And use it in the
code by introducing mxser_must_set_enhance_mode instead of
mxser_disable_must_enchance_mode and mxser_enable_must_enchance_mode.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-41-jslaby@suse.cz


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