mxser: extract mxser_receive_chars_new

Extract the new handling of receive chars (using the MUST chip
enhancements) into a separate function. This eliminates the need for the
intr_old label and makes the code of mxser_receive_chars more compact.
In the next step, we will extract the old handling too.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-16-jslaby@suse.cz


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