Add amp common init function to gather common init setting and finaize.
  - add max98390_init_regs func
  - move amp setting to max98390_init_regs func.
  - removed unneceary setting and finalize common register values.
Signed-off-by: Steve Lee <steves.lee@maximintegrated.com>
Link: https://lore.kernel.org/r/20200611094718.18371-1-steves.lee@maximintegrated.comSigned-off-by: Mark Brown <broonie@kernel.org>