ALSA: nm256: Simplify PM callbacks

ALSA: nm256: Simplify PM callbacks

This is a similar cleanup like the commit [3db084fd0af5: ALSA: fm801:
PCI core handles power state for us].

Since pci_set_power_state(), pci_save_state() and pci_restore_state()
are already done in the PCI core side, so we don't need to it doubly.

Also, pci_enable_device(), pci_disable_device() and pci_set_master()
calls in PM callbacks are superfluous nowadays, too, so get rid of
them as well.

Signed-off-by: Takashi Iwai <tiwai@suse.de>