[media] rc: ir-raw: change type of available_protocols to atomic64_t

Changing available_protocols to atomic64_t allows to get rid of the
mutex protecting access to the variable. This helps to simplify
the code.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>