include: usb_ch9: fix USB_FS_ISO_EP_INTERVAL calculation

Per section 5.6.4 of the USB 2.0 standard, Isochronous Endpoints
are derived by the (2^(bInterval-1) * F) formula. The current
implementation uses the formula intended for Interrupt Endpoints.

Signed-off-by: Victor Brzeski <vbrzeski@gmail.com>