at86rf230: Allow slow GPIO pins for "rstn"

Driver code never touches "rstn" signal in atomic context, so there's
no need to implicitly put such restriction on it by using gpio_set_value
to manipulate it. Replace gpio_set_value to gpio_set_value_cansleep to
fix that.

As a an example of where such restriction might be inconvenient,
consider a hardware design where "rstn" is connected to a pin of I2C/SPI
GPIO expander chip.

Cc: Chris Healy <cphealy@gmail.com>
Signed-off-by: NAndrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: NStefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

drivers/net/ieee802154/at86rf230.c

@@ -1715,9 +1715,9 @@ static int at86rf230_probe(struct spi_device *spi)
 	/* Reset */
 	if (gpio_is_valid(rstn)) {
 		udelay(1);
-		gpio_set_value(rstn, 0);
+		gpio_set_value_cansleep(rstn, 0);
 		udelay(1);
-		gpio_set_value(rstn, 1);
+		gpio_set_value_cansleep(rstn, 1);
 		usleep_range(120, 240);
 	}