usb: gadget: f_fs: fix check in read operation

when copying to iter the size can be different then the iov count,
the check for full iov is wrong and make any read on request which
is not the exactly size of iov to return -EFAULT.

So, just check the success of the copy.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>