net: phy: c45: add genphy_c45_pma_read_ext_abilities() function

Move part of the genphy_c45_pma_read_abilities() code to a separate
function.

Some PHYs do not implement PMA/PMD status 2 register (Register 1.8) but
do implement PMA/PMD extended ability register (Register 1.11). To make
use of it, we need to be able to access this part of code separately.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/20231212054144.87527-2-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>