tty: serial: mpc52xx_uart: make rx/tx hooks return unsigned

All these return bitmasks, so it makes more sense to return unsigned --
this is what a reader and also all the callers expect.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220224111028.20917-2-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>