ASoC: ssm2518: Drop .volatile_reg implementation

The implementation of ssm2518_register_volatile always returns false,
this behavior is the same as no .volatile_reg callback implementation
when cache_type != REGCACHE_NONE.

Signed-off-by: Axel Lin <[e-mail]>
Acked-by: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

sound/soc/codecs/ssm2518.c

···
 	.num_dapm_routes = ARRAY_SIZE(ssm2518_routes),
 };
 
-static bool ssm2518_register_volatile(struct device *dev, unsigned int reg)
-{
-	return false;
-}
-
 static const struct regmap_config ssm2518_regmap_config = {
 	.val_bits = 8,
 	.reg_bits = 8,
 
 	.max_register = SSM2518_REG_DRC_9,
-	.volatile_reg = ssm2518_register_volatile,
 
 	.cache_type = REGCACHE_RBTREE,
 	.reg_defaults = ssm2518_reg_defaults,