iwlegacy: remove redundant pointer sta_priv

Pointer sta_priv is assigned but never read, hence it is redundant
and can be removed. Cleans up clang warning:

drivers/net/wireless/intel/iwlegacy/4965-rs.c:2163:2: warning: Value
stored to 'sta_priv' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>