[media] cx24120: Fix minor style typo in Kconfig

This patch changes DVB-S/DVB-S2 to the more commonly used
DVB-S/S2 in the description for DVB_CX24120

Signed-off-by: Jemma Denson <jdenson@gmail.com>
Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>