USB: Gadget: Initialize wMaxPacketSize if not already set

Currently, for ISO and INT, a protocol driver must chose the value for
wMaxPacketSize arbitrarily. The value may be too low, resulting in lesser
than efficient operation or high enough to not work with all UDC drivers.

Take un-initialized wMaxPacketSize as a hint to provide maximum
possible packetsize for the selected endpoint. The protocol may
then choose a value not bigger than that.

Signed-off-by: Jassi Brar <jassi.brar@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>