serial: convert uart sysrq handling to u8

Propagate u8 from the sysrq code further up to serial's
uart_handle_sysrq_char() and friends.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230712081811.29004-6-jirislaby@kernel.org


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