i40e: Fix static analysis tool warning

This patch fixes a problem where a static analysis tool generates
a warning for "INVARIANT_CONDITION: Expression 'enabled_tc' used
in the condition always yields the same result."

Without this patch, the driver will not pass the static analysis
tool checks without generating warnings.

This patch fixes the problem by eliminating the irrelevant check
and redundant assignment for the value of enabled_tc.

Change-ID: Ia7d44cb050f507df7de333e96369d322e08bf408
Signed-off-by: Dave Ertman <david.m.ertman@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>