ahci: misc cleanups for EM stuff

Make the following EM related cleanups.

* Use msleep(1) instead of udelay(100) and reduce retry count to 5.

* s/MAX_SLOTS/EM_MAX_SLOTS/, s/MAX_RETRY/EM_MAX_RETRY/

* Make EM constants enums as suggested by Jeff.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Milburn <dmilburn@redhat.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>