usb: gadget: f_uac2: fix build warning

commit 913e4a90 ("usb: gadget: f_uac2:
finalize wMaxPacketSize according to bandwidth")
added a possible build warning when calling
min(). In order to fix the warning, we just
make sure to call min_t() and tell that its
arguments should be u16.

Cc: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>