iwlagn: solve sparse warning

Remove local variable that was shadowing another one
sta_priv hasn't changed since the beginning of the function, so don't define
another pointer with the same name to the same variable

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>