powerpc: Fix signedness bug in update_flash_db()

powerpc: Fix signedness bug in update_flash_db()

The "count < sizeof(struct os_area_db)" comparison is type promoted to
size_t so negative values of "count" are treated as very high values
and we accidentally return success instead of a negative error code.

This doesn't really change runtime much but it fixes a static checker
warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>