net: ipv6: Fix IPv6 netmask parsing

It should be possible to specify a netmask when
setting a static IPv6 address.  For example:
setenv ip6addr 2001:cafe:cafe:cafe::100/64

The net_prefix_length and net_ip6 should be updated
properly.

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>
Reviewed-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>