net: smsc911x: request and deassert optional RESET GPIO

On some systems (such as the Qualcomm APQ8060 Dragonboard) the
RESET signal of the SMSC911x is not pulled up by a resistor (or
the internal pull-up that will pull it up if the pin is not
even connected) but instead connected to a GPIO line, so that
the operating system must explicitly deassert RESET before use.

Support this in the SMSC911x driver so this ethernet connector
can be used on such targets.

Notice that we request the line to go logical low (deassert)
whilst the line on the actual component is active low. This
is managed in the respective hardware description when
specifying the GPIO line with e.g. device tree or ACPI. With
device tree it looks like this in one case:

  reset-gpios = <&tlmm 30 GPIO_ACTIVE_LOW>;

Which means that logically requesting the RESET line to be
deasserted will result in the line being driven high, taking
the device out of reset.

Cc: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>