dhcpv6: fix incorrect IA type being printed in syslog

dhcpv6: fix incorrect IA type being printed in syslog

Problem: If a status error code is received, "IA_NA" is always written
to syslog.

Cause: IA header type is not converted from network byte order to host
byte order.

Solution: Use ntohs() to convert to host byte order.

Signed-off-by: Nicolas BESNARD <[email protected]>
Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/106
Signed-off-by: Álvaro Fernández Rojas <[email protected]>