usb: gadget: f_uac2: calculate wMaxPacketSize before endpoint match

Calculate wMaxPacketSize before endpoint matching the
descriptor is found.

This allows audio gadget to be used with controllers
which have a shortage or unavailability of endpoints
that can handle max packet size of 1023 (FS) or 1024
(HS).

With this audio gadget can be used on TI's OMAP-L138 SoC
which has a MUSB HS controller with endpoints having max
packet size much less than 1023 or 1024. See mode_2_cfg in
drivers/usb/musb/musb_core.c

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