net: ptp: fix infinite loop in pkt frag

There was wrong implementation in pkt frag removing and inserting.
This was causing infinite loop in either net_buf_frag_last or
net_pkt_get_len. This happened only when the pkt frag removing
and inserting was executed too fast after sending pkt before
ethernet_send calling net_pkt_get_len.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>