Add a module option to allow the GPR mixer controls to have the full
resolution of the hardware, i.e., 0...2^31-1 instead of 0...100.

Because of bugs in userspace tools like alsactl and alsamixer, this is
not yet enabled by default.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>