dissector: icmpv6: Fix compiler warnings

Fix the following compiler warnings that occur when building with "-W
-Wall -Wextra":

proto_icmpv6.c: In function ‘dissect_icmpv6_mcast_rec’:
proto_icmpv6.c:326:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
proto_icmpv6.c: In function ‘dissect_neighb_disc_ops_15’:
proto_icmpv6.c:537:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
proto_icmpv6.c:542:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
proto_icmpv6.c: In function ‘dissect_neighb_disc_ops_16’:
proto_icmpv6.c:604:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
proto_icmpv6.c: In function ‘dissect_neighb_disc_ops_17’:
proto_icmpv6.c:650:40: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>