ocxl: fix signed comparison with less than zero

Currently the comparison of used < 0 is always false because
uses is a size_t. Fix this by making used a ssize_t type.

Detected by Coccinelle:
drivers/misc/ocxl/file.c:320:6-10: WARNING: Unsigned expression
compared with zero: used < 0

Fixes: 5ef3166e ("ocxl: Driver code for 'generic' opencapi devices")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Acked-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/misc/ocxl/file.c

@@ -277,7 +277,7 @@ static ssize_t afu_read(struct file *file, char __user *buf, size_t count,
 	struct ocxl_context *ctx = file->private_data;
 	struct ocxl_kernel_event_header header;
 	ssize_t rc;
-	size_t used = 0;
+	ssize_t used = 0;
 	DEFINE_WAIT(event_wait);
 
 	memset(&header, 0, sizeof(header));