usb: gadget: f_uvc: Fixed wrong length of H.264 frame descriptor

Corrected the length of the H.264 frame descriptor and
used the correct functions to cast the frame descriptor
parameters.

Change-Id: I5895ebbf81a2a29d31962142c92836cc0a15ae4e
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>