usb: gadget: f_uvc: Fix incorrect frame indexing

Each format descriptor should be followed by frame
descriptors with indexes starting from 1.
The current driver does not reset the frame
index to 1 for every format resulting in
incorrect frame index.
Fix this by resetting the loop variable before
each loop for populating frame descriptors.

Change-Id: I3c3e0ab4808409d059bcbf93d2bb45bbb6efeada
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>