net: shell: Fix the output of route command

Print route information by groupping them for each network
interface like this:

IPv6 routes for interface 0xa8007140
====================================
IPv6 prefix : 2001:db8::212:4b00:0:3/128
        neighbor  : 0xa80065e0
        link addr : 00:12:4B:00:00:00:00:03
IPv6 prefix : 2001:db8::212:4b00:0:1/128
        neighbor  : 0xa8006660
        link addr : 00:12:4B:00:00:00:00:01

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>