staging: kpc2000: prevent underflow in cpld_reconfigure()

This function should not allow negative values of "wr_val".  If
negatives are allowed then capping the upper bound at 7 is
meaningless.  Let's make it unsigned.

Fixes: 7dc7967f ("staging: kpc2000: add initial set of Daktronics drivers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200224103325.hrxdnaeqsthplu42@kili.mountainSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/kpc2000/kpc2000/core.c

@@ -110,10 +110,10 @@ static ssize_t cpld_reconfigure(struct device *dev,
 				const char *buf, size_t count)
 {
 	struct kp2000_device *pcard = dev_get_drvdata(dev);
-	long wr_val;
+	unsigned long wr_val;
 	int rv;
 
-	rv = kstrtol(buf, 0, &wr_val);
+	rv = kstrtoul(buf, 0, &wr_val);
 	if (rv < 0)
 		return rv;
 	if (wr_val > 7)