mxser: correct types for uart variables

In struct mxser_port, all the UART registers and masks, and the xon/xoff
character are stored into int. Let's re-type all these as UART registers
are 8bit (u8) and xon/xoff character is an unsigned char (cc_t in
ktermios).

This save some bytes in memory, but more importantly the change makes it
what it really is.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-19-jslaby@suse.cz


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