ata: libahci: Clean-up the ahci_platform_en/disable_phys functions

The phy_ functions handle the NULL pointer case, so there is no need
to skip them if there is a NULL pointer. Moreover, after the error
label there is already no check on the pointer. This patch removes the
unnecessary tests and brings some consistency.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>