USB: fix up problems in the vtusb driver

Add range check on buffer sizes passed in from user space
(max is 8*PAGE_SIZE) which will work for the most common
spectrometers even at pages as small as 1K.

Add kref to vst device structure to preserve reference to the
usb object until we truly are done with it.

From: Stephen Ware <stephen.ware@eqware.net>
From: Dennis O'Brien <dennis.obrien@eqware.net>
Signed-off-by: Dennis O'Brien <dennis.obrien@eqware.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>