upl: Fix buf array size

upl: Fix buf array size

Size of array buf was incorrect due to sizeof returning the
size of an integer (typically 32 bits) instead of a u64 type
(64 bits). Hence, buf array was shorter than expected.

Signed-off-by: Francois Berder <fberder@outlook.fr>
Reviewed-by: Simon Glass <sjg@chromium.org>