i40e: Fix configure TCs after initial DCB disable

in commit a036244c a fix
was put into place to avoid a kernel panic when a non-
supported traffic class configuration was put into place
and then lldp was enabled/disabled on the link partner
switch.  This fix caused it to be necessary to
unload/reload the driver to reenable DCB once a supported
TC config was in place.

The root cause of the original panic was that the function
i40e_pf_get_default_tc was allowing for a default TC other
than TC 0, and only TC 0 is supported as a default.

This patch removes the get_default_tc function and replaces
it with a #define since there is only one TC supported as
a default.

Change-Id: I448371974e946386d0a7718d73668b450b7c72ef
Signed-off-by: Dave Ertman <david.m.ertman@intel.com>
Tested-by: Ronald Bynoe <ronald.j.bynoe@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>