mdio_bus: handle only single PHY reset GPIO

Commit 4c5e7a2c ("dt-bindings: mdio: Clarify binding document")
declared that a MDIO reset GPIO property should have only a single GPIO
reference/specifier, however the supporting code was left intact, still
burdening the kernel with now apparently useless loops -- get rid of them.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>