usb: core: urb make use of usb_endpoint_maxp_mult

Since usb_endpoint_maxp now returns only lower 11 bits mult
calculation here isn't correct anymore and that breaks webcam
for me. Patch make use of usb_endpoint_maxp_mult instead of
direct calculation.

Fixes: abb62184 ("usb: ch9: make usb_endpoint_maxp() return
       only packet size")

Signed-off-by: Mike Krinkin <krinkin.m.u@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>