ospf6d: replace OSPF6_LSA_SIZE with ospf6_lsa_size

ospf6d: replace OSPF6_LSA_SIZE with ospf6_lsa_size

Dropping the macro enables better compiler type checking.

The macro was not used consistently when reading the lsa size from the
header, so this change also aims to use the replacement inline function
consistently.

Keeping the inline function has (marginal) utility in that it ensures that
the endian conversion is consistently performed.

Signed-off-by: Andrew Cooks <acooks.at.bda@gmail.com>