rtmp: Pass the parameters to do_adobe_auth in the right order

do_adobe_auth takes the parameters in the order "opaque, challenge".

Due to the way they are treated, this didn't matter in the tested
setups though - if both are set, we only use one. In the tested
setups (Wowza and Akamai) either one of them were null or they
were both set to the same value, which is why this worked before.

Signed-off-by: Martin Storsjö <martin@martin.st>