iwlagn: disable PS support for iwlagn

Some issues in PS prevent us from supporting it reliably.

When 4965 goes to sleep it stores some data in host DRAM, reads it back
when device wakes up. In 4965 there is a problem that the data is not
correct when ucode starts using it upon wakeup.

For all iwlagn devices there is a problem where command is sent when PS is
enabled. At the moment there is a locking problem with priv->lock not being
held and thus not requesting nic access correctly.

We disable PS until these issues have been resolved.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>