net: phy: bcm7xx: Correct read from expansion register

Since the driver works in the "legacy" addressing mode, we need to write
to the expansion register (0x17) with bits 11:8 set to 0xf to properly
select the expansion register passed as argument.

Fixes: f68d08c4

 ("net: phy: bcm7xxx: Add EPHY entry for 72165")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230508231749.1681169-1-f.fainelli@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>