At the end of either of the read or write loops len is always zero
and hence the non-zero check on len and return of -EIO is redundant
and can be removed.

Detected by CoverityScan, CID#114293 ("Logically dead code")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>