e1000e: static analysis tools complain of a possible null ptr p dereference

Adding this default case resolves the issue.

v2- Removed "break" in default case based on feedback

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>