ata: ahci_brcm: Avoid writing to read-only registers

This commit makes use of the AHCI_HFLAG_YES_ALPM flag to prevent
the driver from writing to the read-only Host Capability register.

It also sets the AHCI_HFLAG_NO_WRITE_TO_RO flag to prevent the AHCI
library from writing to read-only registers.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>