spi: atmel: Fix scheduling while atomic

A call to clk_get_rate appears to be called in the context of an interrupt,
cache the bus clock for the frequency calculations in transmission.

This fixes a 'BUG: scheduling while atomic' and
'WARNING: CPU: 0 PID: 777 at kernel/sched/core.c:2960 atmel_spi_unlock'

Signed-off-by: Ben Whitten <ben.whitten@lairdtech.com>
Signed-off-by: Steve deRosier <steve.derosier@lairdtech.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-atmel.c

@@ -296,6 +296,7 @@ struct atmel_spi {
 	int			irq;
 	struct clk		*clk;
 	struct platform_device	*pdev;
+	unsigned long		spi_clk;
 
 	struct spi_transfer	*current_transfer;
 	int			current_remaining_bytes;
@@ -865,7 +866,7 @@ static int atmel_spi_set_xfer_speed(struct atmel_spi *as,
 	unsigned long		bus_hz;
 
 	/* v1 chips start out at half the peripheral bus speed. */
-	bus_hz = clk_get_rate(as->clk);
+	bus_hz = as->spi_clk;
 	if (!atmel_spi_is_v2(as))
 		bus_hz /= 2;
 
@@ -1634,6 +1635,9 @@ static int atmel_spi_probe(struct platform_device *pdev)
 	ret = clk_prepare_enable(clk);
 	if (ret)
 		goto out_free_irq;
+
+	as->spi_clk = clk_get_rate(clk);
+
 	spi_writel(as, CR, SPI_BIT(SWRST));
 	spi_writel(as, CR, SPI_BIT(SWRST)); /* AT91SAM9263 Rev B workaround */
 	if (as->caps.has_wdrbt) {