res_srtp: Set all possible flags while selecting the Crypto Suite.

The flags of a previous selection could have been set within the
object 'srtp', for example, when the previous selection returned
failure after setting just 'some' flags. Now, not to clutter the
code, all possible flags are cleared first, and then the selected
flags are set as before.

ASTERISK-28903

Change-Id: I1b9d7aade7d5120244ce7e3a8865518cbd6e0eee