dm_resume() will return success (0) rather than -EINVAL if
!dm_suspended_md() upon retry within dm_resume().

Reset the error code at the start of dm_resume()'s retry loop.
Also, remove a useless assignment at the end of dm_resume().

Fixes: ffcc3936 ("dm: enhance internal suspend and resume interface")
Cc: stable@vger.kernel.org # 3.19+
Signed-off-by: NMinfei Huang <mnghuan@gmail.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>