Bluetooth: has: Fix control point error return code

This fixes the return error code for Preset Write operation that shall
be returned when preset is not writable (or disabled via Kconfig like in
this case).

As per HAS_v1.0; 3.2.2.3 Write Preset Name operation
"If the Writable bit in the Properties field of the preset record which
is identified by the Index parameter is set to 0b0, then the server
shall return an ATT_ERROR_RSP PDU to the ATT_WRITE_REQ PDU with
the Error Code parameter set to Write Name Not Allowed."

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>