ip17xx: correct aneg_done return value

PHY core treats any positive return value as the auto-negotiation done
indication. Since we do not actually check any device register in this
callback then update it to return positive value with a neutral meaning
instead of the register flag to avoid  confusing for future readers.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
(cherry picked from commit 2cc61e6e8e951b1480a024fdb66bc539aa163bbc)