ixgbe: fix return values and memcpy parameters to eliminate Smatch warnings

This patch removes the rval variable returns from function and replaces
them with direct returns in ixgbe_dcbnl_getnumtcs. It also changes how
ixgbe_gstrings_test is copied into data with memcpy in ixgbe_get_strings
because "*ixgbe_gstrings_test too small (32 vs 160)".

Signed-off-by: Josh Hay <joshua.a.hay@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>