kernel: net: phy: realtek: fix NULL pointer dereference

kernel: net: phy: realtek: fix NULL pointer dereference

The previous attempt to replace an open coded paged read in the RealTek
Ethernet PHY driver was too naive and resulted in breaking the r8169
PCIe Ethernet driver which also makes use of the RealTek Ethernet PHY
driver.
Fix this by instead of using the (not yet populated) paged operations
rather use rtl821x_write_page and protect the whole paged read operation
using the MDIO bus mutex.

Fixes: 998b973157 ("kernel: net: phy: realtek: improve RealTek 2.5G PHY driver")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>