usb: gadget: f_fs: rename descriptor parsing functions

ffs_do_desc() handles one descriptor, while ffs_do_descs() handles a number
of descriptors. The tho names are so similar that it causes confusion.
Rename to reflect their purpose better.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>