Remove encal_dl_type from struct flow

There were plans to use this in conjunction with inner/outer flows,
however that plan has been changed in favour of using recirculation.
This leaves us with the current usage.

encal_dl_type is currently only used to allow decoding of packets used in
the test suite. However, this is a bit of a fudge and the packets may be
provided as hexadecimal instead.

Also remove comments from parse_l2_5_onward() relating to MPLS which are
not in keeping with the commenting throughout the rest of the function.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Jesse Gross <jesse@nicira.com>