usbaudio: Fix output device routing

Fix issue with commit a35e783 causing the output device to reset back
to the hard coded value of CARD_ID after a new output device is
specified.

This happens because out_set_parameters() receives a single "routing"
key/value pair just after setting a new output device. The if/else
statement would then cause a reset of the output device back to CARD_ID
because the "routing" message does not contain a card or device id.

Change-Id: Ib7c7eb7ef675ae371033a7ae7ce73782cac2c898