lib: Add missing enum values in switch statement for if_link_type_str

lib: Add missing enum values in switch statement for if_link_type_str

The switch statement over `enum zebra_link_type` had a default
and FRR was missing a few of the pre-defined types we cared about.
Remove the default statement and add the missing values.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>