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>