ixgbe: only compile ixgbe_debugfs.o when enabled

This patch modifies ixgbe_debugfs.c and the Makefile for the ixgbe
driver to only compile the file when the config is enabled. This means
we can remove the #ifdef inside the ixgbe_debugfs.c file.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>