i40e: removed unreachable code

Removed some of unnecessary if statements and unreachable code found by
static code analysis tool.
The return value of i40e_vsi_control_rings(..., false) is always 0. So,
test for non-zero will never be true. The function has been split into
"int i40e_vsi_start_rings()" and "void i40e_vsi_stop_rings()" for better
understanding.
Similarly, the function i40e_vsi_kill_vlan() never fails. So, checking
for return value is also unnecessary. Function definition changed to void.
The i40e_loopback_test() function is not implemented. The function and
all references to loopback testing were removed.

Change-ID: Id45cf66f6689ce2bc4e887de13f073e30e8431bd
Signed-off-by: Filip Sadowski <filip.sadowski@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>