media: flexcop-usb: ensure -EIO is returned on error condition

An earlier commit hard coded a return 0 to function flexcop_usb_i2c_req
even though the an -EIO was intended to be returned in the case where
ret != buflen.  Fix this by replacing the return 0 with the return of
ret to return the error return code.

Addresses-Coverity: ("Unused value")

Fixes: b430eaba ("[media] flexcop-usb: don't use stack for DMA")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>