iwlwifi: define firmware file name once

string repeated for definition of ucode firmware file and in preparation for
multiple firmware files need to consolidate location for their definition.

Signed-off-by: Jay Sternberg <jay.e.sternberg@intel.com>
Acked-by: Tomas Winkler <tomas.winler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>