iwlwifi: pcie: enable oscillator for L1 exit

Enabling the oscillator consumes slightly more power (100uA)
but allows to make sure that we exit from L1 on time.

Not doing so might lead to a PCIe specification violation
since we might wake up from L1 at the wrong time.
This issue has been identified on 3160 and 7260 only.
On older NICs L1 off is not enabled, on newer NICs (7265),
the issue is fixed.

When the bug occurs the user sees that the NIC has
disappeared from the PCI bridge, any access to the device
returns 0xff.

This fixes:
	https://bugzilla.kernel.org/show_bug.cgi?id=64541

and has been extensively discussed here:
	http://markmail.org/thread/mfmpzqt3r333n4bo



Cc: stable@vger.kernel.org [3.10+]
Fixes: 99cd4714 ("iwlwifi: add 7000 series device configuration")
Reported-and-tested-by: wzyboy <wzyboy@wzyboy.org>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>