ofproto-dpif: Correct endian problem in recv_upcalls()

ofproto-dpif: Correct endian problem in recv_upcalls()

Use nl_attr_get_u32() instead of nl_attr_get_be32() to parse nla
so that the decoded value which is passed to mhash_add()
is host byte order as mhash_add() expects.

This resolves a minor regression introduced by
10e576406c7444ef ("ofproto-dpif: Move special upcall handling into
ofproto-dpif-upcall.").

I do not expect this change has any runtime implications.

Detected using sparse.

Cc: Ethan Jackson <ethan@nicira.com>
Cc: Ben Pfaff <blp@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>