net: phy: realtek: fix RTL8211F interrupt mode

After commit b94d22d9 "ARM64: dts: meson-gx: add external PHY
interrupt on some platforms" ethernet stopped working on my Odroid-C2
which has a RTL8211F phy.

It turned out that no interrupts were triggered. Further analysis
showed the register INER can't be altered on page 0.
Because register INSR needs to be accessed via page 0xa43 I assumed
that register INER needs to be accessed via some page too.
Some brute force check resulted in page 0xa42 being the right one.

With this patch the phy is working properly in interrupt mode.

Fixes: 3447cf2e ("net/phy: Add support for Realtek RTL8211F")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>