davinci: cleanup davinci_sync_env_enetaddr() fucntion

check for the return status for eth_getenv_enetaddr_by_index()
and eth_setenv_enetaddr() functions and print appropriate message
on failure.

Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Cc: Tom Rini <trini@ti.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>