Apply FreeBSD ix-3.3.6.tar.gz's change to NetBSD. Tested on C3000 and X550-T1,
but not tested on Xeon D:
- Add firmware recovery mode for X550, X550A(Xeon D) and X550EM (C3000):
  - FreeBSD always set IXGBE_FEATURE_RECOVERY_MODE without checking the
    NVM image version. We compare it against 2.0 to not to make new callout and
    not to call extra atomic operations.
  - In some FreeBSD's sysctl functions, atomic_load_acq_int() is called
    before a null pointer check. We call it after null pointer check.
  - Before calling atomic_load_acq_uint(), check adapter->feat_en flags
    to save atomic operation call.
  - We don't check recovery_mode in ixgbe_set_sysctl_value() because this
    function doesn't touch any hardware register.
  - NetBSD don't have FreeBSD's atomic_load_acq_int()-like function, so do it
    with membar_sync(). Thanks riastradh@ for the advice.
- FreeBSD's ix-3.3.6 changed ixgbe_enable_aim from TRUE to FALSE, but we will
  keep it as TRUE because we have already fixed some bugs.
- Remove IXGBE_DEV_ID_82599_LS(0x154f) support again. I don't know why. This
  was added in ix-3.2.18.tar.gz(NetBSD: ixgbe_82599.c rev. 1.20) and removed in
  ix-3.3.6.tar.gz.
- On X550EMU, use ixgbe_identify_sfp_module_X550em() instead of
  ixgbe_identify_module_generic(). ixgbe_identify_sfp_module_X550em() has
  extra check (e.g. exclude 1G copper).
- if_sriov.c's change doesn't affect to NetBSD because we don't support
  SR-IOV PF function.

(msaitoh)