[ALSA] usb-audio - avoid unnecessary double buffering

USB generic driver
When a USB packet boundary falls exactly on a buffer boundary, hwptr
remains at the end of the buffer which causes an overflow in the next
iteration and triggers double buffering although the next packet would
actually be contiguous.

This patch ensures that hwptr is always smaller than buffer_size.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>