omap3evm: eth: Include functions only when necessary

These functions are not required when CONFIG_CMD_NET
is not defined:
  - setup_net_chip()
  - reset_net_chip()
  - board_eth_init()

This patch wraps them in #ifdef CONFIG_CMD_NET...#endif

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>