[nrf fromtree] 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 <[email protected]>
(cherry picked from commit d9c7b19)
Signed-off-by: Tomasz Moń <[email protected]>

Author: vbrzeski

include/zephyr/usb/usb_ch9.h

@@ -354,8 +354,8 @@ struct usb_association_descriptor {
 /** Calculate high speed interrupt endpoint bInterval from a value in microseconds */
 #define USB_HS_INT_EP_INTERVAL(us)	CLAMP((ilog2((us) / 125U) + 1U), 1U, 16U)
 
-/** Calculate high speed isochronous endpoint bInterval from a value in microseconds */
-#define USB_FS_ISO_EP_INTERVAL(us)	CLAMP(((us) / 1000U), 1U, 16U)
+/** Calculate full speed isochronous endpoint bInterval from a value in microseconds */
+#define USB_FS_ISO_EP_INTERVAL(us)	CLAMP((ilog2((us) / 1000U) + 1U), 1U, 16U)
 
 /** Calculate high speed isochronous endpoint bInterval from a value in microseconds */
 #define USB_HS_ISO_EP_INTERVAL(us)	CLAMP((ilog2((us) / 125U) + 1U), 1U, 16U)