ofp-print: Use ds_chomp() in ofp10_match_to_string()

Slightly simplify ofp10_match_to_string() by using ds_chomp()
rather than open-coding its logic.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>