iwlwifi: Fix synchronous host command

This patch replaces static variable from send_cmd_sync
with flag in priv->status. It was used for reentrance protection
but clearly made it impossible to stuck more cards into the same machine

In addition it force check of return values of synchronous commands
commands that doesn't requires return value async commands have to be used

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Yi Zhu <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>