bluetooth: audio: micp: Wrong error response for invalid Mute Value.

Qualification test MICS/SR/SPE/BI-01-C [Invalid Mute Value]

Specification: Microphone Control Service Revision v1.0 Section 3.1.1

If the client writes a value of Disabled or RFU to the Mute
characteristic, the server shall return an ATT Error Response with the
ATT error code Value Not Allowed (0x13) as defined in Assigned Numbers.

Signed-off-by: Henrik Eriksen <heri@demant.com>