[ipv6] Ignore SLAAC on prefixes with an incompatible prefix length

[ipv6] Ignore SLAAC on prefixes with an incompatible prefix length

Experience suggests that routers are often misconfigured to advertise
SLAAC even on prefixes that do not have a SLAAC-compatible prefix
length.  iPXE will currently treat this as an error, resulting in the
prefix being ignored completely.

Handle this misconfiguration by ignoring the autonomous address flag
when the prefix length is unsuitable for SLAAC.

Reported-by: Malte Janduda <mail@janduda.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>