realtek: clean up rtl838x MDIO busy wait loop

realtek: clean up rtl838x MDIO busy wait loop

Don't use udelay to allow other kernel tasks to execute if the kernel
has been built without preemption. Also determine the timeout based on
jiffies instead of loop iterations.

This is especially important on devices containing a watchdog with a
short timeout. Without this change, the watchdog is not serviced during
PHY patching which can take multiple seconds.

Tested-by: Birger Koblitz <mail@birger-koblitz.de>
Signed-off-by: Jan Hoffmann <jan@3e8.eu>