media: uvcvideo: Simplify uvc_endpoint_max_bpi()

The case USB_SPEED_WIRELESS and the default one were doing the same.

Also, make always use of usb_endpoint_maxp_mult, as it should have a
sane value, even for LOW speed and WIRELESS.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>