Bluetooth: BAP: SD: Add missing error checks for mutex locks

Add missing error checks for mutex locks after the mutexes
were changed to not use K_FOREVER which should not be done in
the BT RX thread.

A larger overhaul of how mutexes are using within the scan delegator
should be considered, as there are a lot of locks and unlocks when
handling the callbacks.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>