ahci: mmio_base is a virtual address

Don't store it in a u32.

Don't dereference the bus address as if it were a virtual address
(fixes 284231e4 ("ahci: Support splitting of read transactions
into multiple chunks")).

Fixes crash on boot in MPC8641HPCN_36BIT target.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Vadim Bendebury <vbendeb@chromium.org>
Acked-by: York Sun <yorksun@freescale.com>