iwlagn: Add missing comma between constant string array

Multiple quoted strings are concatenated without comma separators.

Make the array const while there.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>