usb: ch9: make usb_endpoint_maxp() return only packet size

Now that we have a helper to gather periodic
endpoints' multiplier bits from wMaxPacketSize and
every driver is using it, we can safely make sure
that usb_endpoint_maxp() returns only bits 10:0 of
wMaxPacketSize which is where the actual packet size
lies.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>