PCI: armada8k: Remove useless test before clk_disable_unprepare()

clk_disable_unprepare() already checks that the clock pointer is valid.
No need to test it before calling it.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>