serial: make uart_insert_char() accept u8s

Both the character and flag are 8-bit values. So switch from unsigned
ints to u8s. The drivers will be cleaned up in the next round.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230712081811.29004-7-jirislaby@kernel.org


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