media: stv0288: use explicitly signed char

With char becoming unsigned by default, and with `char` alone being
ambiguous and based on architecture, signed chars need to be marked
explicitly as such. Use `s8` and `u8` types here, since that's what
surrounding code does. This fixes:

drivers/media/dvb-frontends/stv0288.c:471 stv0288_set_frontend() warn: assigning (-9) to unsigned variable 'tm'
drivers/media/dvb-frontends/stv0288.c:471 stv0288_set_frontend() warn: we never enter this loop

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: linux-media@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>