iwlwifi: keep the STATUS_EXIT_PENDING flag till the end of down flow

This patch avoids unsetting STATUS_EXIT_PENDING in the middle of the
down flow.

Signed-off-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>