bnxt_en: Fix VLAN acceleration handling in bnxt_fix_features().

The current logic in bnxt_fix_features() will inadvertently turn on both
CTAG and STAG VLAN offload if the user tries to disable both.  Fix it
by checking that the user is trying to enable CTAG or STAG before
enabling both.  The logic is supposed to enable or disable both CTAG and
STAG together.

Fixes: 5a9f6b23

 ("bnxt_en: Enable and disable RX CTAG and RX STAG VLAN acceleration together.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>