PDM: fix setGain() for dfsdm #1166

Job	Run time
compile-examples (arduino:mbed:nano33ble)	8m 7s
compile-examples (arduino:mbed:envie_m7)	17m 21s
compile-examples (arduino:mbed:nanorp2040connect)	13m 13s
compile-examples (arduino:mbed:nicla_sense)	13m 59s
compile-examples (arduino:mbed:nicla_vision)	11m 51s
compile-examples (arduino:mbed:opta)	15m 54s
compile-examples (arduino:mbed:giga)	24m 25s
compile-examples (arduino:mbed:nicla_voice)	5m 22s
compile-examples (arduino:mbed:edge_control)	5m 45s
compile-examples (arduino:mbed:envie_m7:target_core=cm4, - libraries/KernelDebug - libraries/Port...	4m 28s
	2h 0m 25s

Provide feedback