ASoC: Avoid a redundant read in cs42l51_pdn_event

snd_soc_update_bits already does read-modify-write,
no need to read the register before calling snd_soc_update_bits.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>