spi: microchip-core: fix passing zero to PTR_ERR warning

It is possible that the error case for devm_clk_get() returns NULL,
in which case zero will be passed to PTR_ERR() as shown by the Smatch
static checker warning:
drivers/spi/spi-microchip-core.c:557 mchp_corespi_probe()
warn: passing zero to 'PTR_ERR'

Remove the NULL check and carry on with a dummy clock in case of an
error. To avoid a potential div zero, abort calculating clkgen if
clk_get_rate(spi->clk) is zero.

Fixes: 9ac8d176 ("spi: add support for microchip fpga spi controllers")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/linux-spi/20220615091633.GI2168@kadam/Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220615142028.2991915-1-conor.dooley@microchip.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-microchip-core.c

@@ -433,6 +433,8 @@ static int mchp_corespi_calculate_clkgen(struct mchp_corespi *spi,
 	unsigned long clk_hz, spi_hz, clk_gen;
 
 	clk_hz = clk_get_rate(spi->clk);
+	if (!clk_hz)
+		return -EINVAL;
 	spi_hz = min(target_hz, clk_hz);
 
 	/*
@@ -553,7 +555,7 @@ static int mchp_corespi_probe(struct platform_device *pdev)
 	}
 
 	spi->clk = devm_clk_get(&pdev->dev, NULL);
-	if (!spi->clk || IS_ERR(spi->clk)) {
+	if (IS_ERR(spi->clk)) {
 		ret = PTR_ERR(spi->clk);
 		dev_err(&pdev->dev, "could not get clk: %d\n", ret);
 		goto error_release_master;