iwlwifi: fix firmware filename for 3160

The MODULE_FIRMWARE() for 3160 should be using the 7260 version as
it's done in the device configuration struct instead of referencing
IWL3160_UCODE_API_OK which doesn't even exist.

Cc: <stable@vger.kernel.org> [3.8+]
Reported-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>