ahci: cosmetics and cleanup

- print the correct speed
- print all the AHCI capability flags
(information taken from Linux kernel driver)
- clean up some comments

For example, this might show the following string:
AHCI 0001.0300 32 slots 6 ports 6 Gbps 0x3 impl SATA mode

Signed-off-by: Stefan Reinauer <reinauer@chromium.org>

Commit-Ready: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Stefan Reinauer <reinauer@chromium.org>