media: usbtv: core: make use of new usb_endpoint_maxp_mult()

We have introduced a helper to calculate multiplier
value from wMaxPacketSize. Start using it.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: <linux-media@vger.kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>