net: mdio-gpio: support access that may sleep

Some systems using mdio-gpio may use gpio on message based busses, which
require sleeping (e.g. gpio from an I2C I/O expander).

Since this driver does not use IRQ handler, it is safe to use the
_cansleep suffixed gpio accessors.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>