Fix some issues found by a static checker:

When allocating an AFU interrupt, if the driver cannot copy the output
parameters to userland, the errno value was not set to EFAULT

Remove a (now) useless cast.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NFrederic Barrat <fbarrat@linux.vnet.ibm.com>
Acked-by: NAndrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>