usb: gadget: f_uac2: fix calculation of uac2->p_interval

The p_interval should be less if the 'bInterval' at the descriptor
is larger, eg, if 'bInterval' is 5 for HS, the p_interval should be
8000 / 16 = 500.

It fixes the patch 9bb87f16 ("usb: gadget: f_uac2: send
reasonably sized packets")

Cc: <stable@vger.kernel.org> # v3.18+
Fixes: 9bb87f16 ("usb: gadget: f_uac2: send reasonably sized packets")
Acked-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>