drm/amdgpu: Prevent shift wrapping in amdgpu_read_mask()

If the user passes a "level" value which is higher than 31 then that
leads to shift wrapping.  The undefined behavior will lead to a
syzkaller stack dump.

Fixes: 5632708f ("drm/amd/powerplay: add dpm force multiple levels on cz/tonga/fiji/polaris (v2)")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/pm/amdgpu_pm.c

@@ -1095,7 +1095,7 @@ static ssize_t amdgpu_get_pp_dpm_sclk(struct device *dev,
 static ssize_t amdgpu_read_mask(const char *buf, size_t count, uint32_t *mask)
 {
 	int ret;
-	long level;
+	unsigned long level;
 	char *sub_str = NULL;
 	char *tmp;
 	char buf_cpy[AMDGPU_MASK_BUF_MAX + 1];
@@ -1110,8 +1110,8 @@ static ssize_t amdgpu_read_mask(const char *buf, size_t count, uint32_t *mask)
 	tmp = buf_cpy;
 	while ((sub_str = strsep(&tmp, delimiter)) != NULL) {
 		if (strlen(sub_str)) {
-			ret = kstrtol(sub_str, 0, &level);
-			if (ret)
+			ret = kstrtoul(sub_str, 0, &level);
+			if (ret || level > 31)
 				return -EINVAL;
 			*mask |= 1 << level;
 		} else