ahci: move AHCI_HFLAGS() macro to ahci.h

We will need this macro in both ahci.c and ahci_platform.c, so just move it
to the header.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Cc: stable@kernel.org