usb: gadget: composite: correctly initialize ep->maxpacket

usb_endpoint_maxp() returns wMaxPacketSize in its
raw form. Without taking into consideration that it
also contains other bits reserved for isochronous
endpoints.

This patch fixes one occasion where this is a
problem by making sure that we initialize
ep->maxpacket only with lower 10 bits of the value
returned by usb_endpoint_maxp(). Note that seperate
patches will be necessary to audit all call sites of
usb_endpoint_maxp() and make sure that
usb_endpoint_maxp() only returns lower 10 bits of
wMaxPacketSize.

Cc: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>