sound: usb-audio: remove MIN_PACKS_URB

Remove the MIN_PACKS_URB symbol because other limits can force the
number of packets down to one, regardless of the value of this symbol,
and nobody has ever changed it anyway.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>