Bluetooth: ascs: Fix ASE Control Point PDU length validation

This moves ASE Control PDU length validation before request processing.
If the PDU length is not as expected, the operation shall be considered
as invalid. Thus shall be rejected.

ASCS_v1.0; 5 ASE Control operations
"A client-initiated ASE Control operation shall be defined as an invalid
length operation if the total length of all parameters written by the
client is not equal to the total length of all fixed parameters plus
the length of any variable length parameters for that operation"

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