net: gptp: fix comparsion of stepsRemoved

The original implementation select the port
as best_port when stepsRemoved is equal to
the one of best_vector.
When challenger is from announce message
set in GPTP_PA_INFO_SUPERIOR_MASTER_PORT,
its stepsRemoved is the same as in the message.
When challenger is from master_priority
copied in GPTP_PA_INFO_UPDATE,
its stepsRemoved is [stepsRemoved of bestVector]+1.
Thus, we should add one to the challenger's
stepsRemoved in comparsion to avoid useless
update of bestVector.

Signed-off-by: Yusuke Omae <s1413091@gmail.com>