clk: vexpress: NULL dereference on error path

If the allocation fails then we dereference the NULL in the error path.
Just return directly.

Fixes: ed27ff1d ('clk: Versatile Express clock generators ("osc") driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>