Requiring LLDs to format multiple error description messages properly
doesn't work too well. Help LLDs a bit by making ata_ehi_push_desc()
insert ", " on each invocation. __ata_ehi_push_desc() is the raw
version without the automatic separator.
While at it, make ehi_desc interface proper functions instead of
macros.
Signed-off-by: Tejun Heo <htejun@gmail.com> Signed-off-by: Jeff Garzik <jeff@garzik.org>