tty: mmc: sdio: use u8 for flag

Switch character types to u8. To conform to characters in the rest of
the tty layer.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: linux-mmc@vger.kernel.org
Link: https://lore.kernel.org/r/20231206073712.17776-17-jirislaby@kernel.org


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