phy: Move PHY_INTERFACE_MODE_NA to the beginning of the enum definition

Move PHY_INTERFACE_MODE_NA to the beginning of the enum definition to
make it have zero value. This makes it possible (although not
encouraged) to test for invalid/nonexistent interface mode with !val
instead of val == PHY_INTERFACE_MODE_NA.

The comment near the definition says "Must be last", because when the
constant was introduced in commit 5f184715 ("Create PHY Lib for
U-Boot"), it was used as the maximum value when interating over the
constants. But this is no longer true - we use PHY_INTERFACE_MODE_MAX
for that now, and so we can move it.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>