net: reject Bootp/DHCP packets with bad OP value

Rename check_packet to check_reply_packet to make its function more
obvious.
The check for DHCP_* values is completely off, as it should
compare against DHCP option 53 (Message Type). Only valid value for
any Bootp/DHCP reply is BOOTREPLY.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>