hwmon: (nct7904) Correct divide by 0

We hit a kernel panic due to a divide by 0 in nct7904_read_fan() for
the hwmon_fan_min case. Extend the check to hwmon_fan_input case as well
for safety.

[ 1656.545650] divide error: 0000 [#1] SMP PTI
[ 1656.545779] CPU: 12 PID: 18010 Comm: sensors Not tainted 5.4.47 #1
[ 1656.546065] RIP: 0010:nct7904_read+0x1e9/0x510 [nct7904]
...
[ 1656.546549] RAX: 0000000000149970 RBX: ffffbd6b86bcbe08 RCX: 0000000000000000
...
[ 1656.547548] Call Trace:
[ 1656.547665]  hwmon_attr_show+0x32/0xd0 [hwmon]
[ 1656.547783]  dev_attr_show+0x18/0x50
[ 1656.547898]  sysfs_kf_seq_show+0x99/0x120
[ 1656.548013]  seq_read+0xd8/0x3e0
[ 1656.548127]  vfs_read+0x89/0x130
[ 1656.548234]  ksys_read+0x7d/0xb0
[ 1656.548342]  do_syscall_64+0x48/0x110
[ 1656.548451]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: d65a5102 ("hwmon: (nct7904) Convert to use new hwmon registration API")
Signed-off-by: Jason Baron <jbaron@akamai.com>
Link: https://lore.kernel.org/r/1598026814-2604-1-git-send-email-jbaron@akamai.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/nct7904.c

@@ -231,7 +231,7 @@ static int nct7904_read_fan(struct device *dev, u32 attr, int channel,
 		if (ret < 0)
 			return ret;
 		cnt = ((ret & 0xff00) >> 3) | (ret & 0x1f);
-		if (cnt == 0x1fff)
+		if (cnt == 0 || cnt == 0x1fff)
 			rpm = 0;
 		else
 			rpm = 1350000 / cnt;
@@ -243,7 +243,7 @@ static int nct7904_read_fan(struct device *dev, u32 attr, int channel,
 		if (ret < 0)
 			return ret;
 		cnt = ((ret & 0xff00) >> 3) | (ret & 0x1f);
-		if (cnt == 0x1fff)
+		if (cnt == 0 || cnt == 0x1fff)
 			rpm = 0;
 		else
 			rpm = 1350000 / cnt;