net: phy: mdio-gpio: Fix working over slow can_sleep GPIOs

This commit re-enables support for slow GPIO pins. It was initially
introduced by commit 2d6c9091 ("net: mdio-gpio: support access that
may sleep") and got lost by commit 7e5fbd1e ("net: mdio-gpio:
Convert to use gpiod functions where possible").

Also add a warning about slow GPIO pins like it is done in i2c-gpio.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: David S. Miller <davem@davemloft.net>