ALSA: PCI: Remove superfluous pci_set_drvdata(pci, NULL) at remove

As drvdata is cleared to NULL at probe failure or at removal by the
driver core, we don't have to call pci_set_drvdata(pci, NULL) any
longer in each driver.

The only remaining pci_set_drvdata(NULL) is in azx_firmware_cb() in
hda_intel.c.  Since this function itself releases the card instance,
we need to clear drvdata here as well, so that it won't be released
doubly in the remove callback.

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