irqchip/ti-sci: Simplify with dev_err_probe()

Common pattern of handling deferred probe can be simplified with
dev_err_probe().  Less code and the error value gets printed.

There is also no need to assign NULL to 'intr->sci' as it is part of
devm-allocated memory.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200902174615.24695-1-krzk@kernel.org