net: phy: Unpublish phy_connect_dev()

The phy_connect_dev() is legacy API, now that there are no users,
make it internal to phy.c and unpublish it from headers.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>