ahci_xgene: Fix the error print invalid resource for APM X-Gene SoC AHCI SATA Host...

This patch fixes the error print invalid resource for the APM X-Gene
SoC AHCI SATA Host Controller driver. This print was due to the fact
that the controller 3 don't have a mux resource. This didn't result
in any errors but the print seems like meaningless.

Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Suman Tripathi <stripathi@apm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>