[media] cx88: constify mb86a16_config structure

Declare mb86a16_config structure as const as it is only passed as
an argument to the function dvb_attach. dvb_attach calls its first
argument on the rest of its arguments. The first argument of
dvb_attach in the changed case is mb86a16_attach and the parameter of
this function to which the object reference is passed is of type
const. So, mb86a16_config structures having this property can be made
const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>