iwl3945: Reset saved POWER_TABLE_CMD in "up"

Power-saving logic will not re-issue a POWER_TABLE_CMD if a new command
matches the prior one.  This can be bad if we re-start the device due to
e.g. uCode error; the new POWER_TABLE_CMD (required to invoke power-saving)
may match the prior POWER_TABLE_CMD issued before the uCode error.

Ensure the POWER_TABLE_CMD is sent to device when uCode is up.

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>