net/: sparse fixes

bootp.c:44:14: warning: symbol 'dhcp_state' was not declared. Should it be static?
bootp.c:45:15: warning: symbol 'dhcp_leasetime' was not declared. Should it be static?
bootp.c:46:10: warning: symbol 'NetDHCPServerIP' was not declared. Should it be static?
arp.c:30:17: warning: symbol 'NetArpWaitReplyIP' was not declared. Should it be static?
arp.c:37:16: warning: symbol 'NetArpTxPacket' was not declared. Should it be static?
arp.c:38:17: warning: symbol 'NetArpPacketBuf' was not declared. Should it be static?
atheros.c:33:19: warning: symbol 'AR8021_driver' was not declared. Should it be static?
net.c:183:7: warning: symbol 'PktBuf' was not declared. Should it be static?
net.c:159:21: warning: symbol 'net_state' was not declared. Should it be static?
ping.c:73:6: warning: symbol 'ping_start' was not declared. Should it be static?
ping.c:82:13: warning: symbol 'ping_receive' was not declared. Should it be static?
tftp.c:53:7: warning: symbol 'TftpRRQTimeoutMSecs' was not declared. Should it be static?
tftp.c:54:5: warning: symbol 'TftpRRQTimeoutCountMax' was not declared. Should it be static?
eth.c:125:19: warning: symbol 'eth_current' was not declared. Should it be static?

Note: in the ping.c fix, commit a36b12f9
"net: Move PING out of net.c" mistakenly carried the ifdef CMD_PING
clause from when it was necessary to avoid warnings when it was embedded
in net.c.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>