lldp: Get rid of POKE macros in favor of inline functions.

lldp: Get rid of POKE macros in favor of inline functions.

The POKE macros previously used here don't match the style usually used in
OVS and they require the user to know exactly how many bytes to reserve.
This commit replaces them by easier-to-use inline functions that take
advantage of the ofpbuf interface.

Also removes a few PEEK macros that weren't used anywhere.

Signed-off-by: Ben Pfaff <blp@nicira.com>