iwlegacy: fix antenna bitmask

This patch is based on "iwlwifi: fix antenna bitmask".
(362b0563)

Like the new iwlagn devices, the old 4965N device only
supports a maximum of three antennas. Hence only three
bits are used, the fourth bit is likely the A-MPDU
indicator.

Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>