ALSA: bt87x: Make load_all parameter working again

Along with a clean up commit [e9f66d9b: ALSA: pci: clean up using
module_pci_driver()], bt87x driver lost the functionality of load_all
parameter.  This patch does a partial revert of the commit only for
bt87x.c to recover it.

Reported-by: Clemens Ladisch <cladisch@googlemail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>