i40e: use changed_flags to check I40E_FLAG_DISABLE_FW_LLDP

i40e: use changed_flags to check I40E_FLAG_DISABLE_FW_LLDP

Currently in i40e_set_priv_flags we use new_flags to check for the
I40E_FLAG_DISABLE_FW_LLDP flag.  This is an issue for a few a reasons.
DISABLE_FW_LLDP is persistent across reboots/driver reloads.  This means
we need some way to detect if FW LLDP is enabled on init.  We do this by
trying to init_dcb and if it fails with EPERM we know LLDP is disabled
in FW.

This could be a problem on older FW versions or NPAR enabled PFs because
there are situations where the FW could disable LLDP, but they do _not_
support using this flag to change it.  If we do end up in this
situation, the flag will be set, then when the user tries to change any
priv flags, the driver thinks the user is trying to disable FW LLDP on a
FW that doesn't support it and essentially forbids any priv flag
changes.

The fix is simple, instead of checking if this flag is set, we should be
checking if the user is trying to _change_ the flag on unsupported FW
versions.

This patch also adds a comment explaining that the cmpxchg is the point
of no return.  Once we put the new flags into pf->flags we can't back
out.

Signed-off-by: Alan Brady <alan.brady@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>