doc: Fix eth_env_[gs]et_enetaddr() return value

Per env/common.c, eth_env_get_enetaddr() returns the same return
values as is_valid_ethaddr(), i.e. true if valid, false otherwise.
Per env/common.c, eth_env_set_enetaddr() may return -EEXIST is the
ethaddr is already set. Fix both.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>