ahci_platform: make structs static

These structs are used only for ahci_platform.c, so they should be
static. Thanks to Fengguang for the (automated) suggestion.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>