[media] rc-core.h: Surround macro with do {} while (0)

Macros coded with if statements should be do { if... } while (0)
so the macros can be used in other if tests.

Use ##__VA_ARGS__ for variadic macro as well.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>