libahci_platform: Staticize ahci_platform_<en/dis>able_phys()

ahci_platform_enable_phys() and ahci_platform_disable_phys() are currently
exported, but they are not used anywhere else other than libahci_platform.c.

So make them static and do not export them to fix the following sparse
warnings:

drivers/ata/libahci_platform.c:52:5: warning: symbol 'ahci_platform_enable_phys' was not declared. Should it be static?
drivers/ata/libahci_platform.c:88:6: warning: symbol 'ahci_platform_disable_phys' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Tejun Heo <tj@kernel.org>