phy: bcm-ns-usb3: always wait for idle after writing to the PHY reg

Move MDIO specific code to the writing helper function. This makes init
code a bit more generic and doesn't require it to track what happens
after every write.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>