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

[ Upstream commit df5a8ec64eed7fe45b556cfff503acd6429ab817 ]

Up until commit 7e5fbd1e ("net: mdio-gpio: Convert to use gpiod
functions where possible"), the _cansleep variants of the gpio_ API was
used. After that commit and the change to gpiod_ API, the _cansleep()
was dropped. This then results in WARN_ON() when used with GPIO
devices which do sleep. Add back the _cansleep() to avoid this.

Fixes: 7e5fbd1e ("net: mdio-gpio: Convert to use gpiod functions where possible")
Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>