powerpc/powernv/npu: Allocate enough memory in pnv_try_setup_npu_table_group()

There is a typo so we accidentally allocate enough memory for a pointer
when we wanted to allocate enough for a struct.

Fixes: 0bd97167

 ("powerpc/powernv/npu: Add compound IOMMU groups")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>