usb: gadget: uvc: cleanup UVCG_FRAME_ATTR macro

1) Change "conv" an "vnoc" to "to_cpu_endian" to "to_little_endian".
2) No need to check the "limit" because that is already handled in
   kstrtoXX so delete that parameter along with the check.
3) By using a "bits" parameter, we can combine the "uxx" parameter and
   the "str2u" parameters.
4) The kstrtou##bits() conversion does not need to be done under the
   mutex so move it to the start of the function.
5) Change the name of "identity_conv" to "noop_conversion".

Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>