net/eth.c: Fix env_enetaddr signed overflow

Assigning the output of simple_strtoul(CB:A9:87:65:43:21) to `char', we are
warned as below:

  U-Boot 1.2.0 (Aug 30 2007 - 08:27:37)

  DRAM:  256 MB
  Flash: 32 MB
  In:    serial
  Out:   serial
  Err:   serial
  Net:   NEC-Candy
  Warning: NEC-Candy MAC addresses don't match:
  Address in SROM is         00:00:4C:80:92:A2
  Address in environment is  FFFFFFCB:FFFFFFA9:FFFFFF87:65:43:21

This patch changes env_enetaddr type from `char' to `unsigned char'.

Cc: Masaki Ishikawa <ishikawa-masaki@cnt.mxe.nes.nec.co.jp>
Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>