libahci: wrong comments in ahci_do_softreset()

AHCI 1.3.1 Spec says that software shall build two H2D register
FISes in the command list to send a software reset.
The comments in ahci_do_softreset() is currently D2H instead of H2D.

Signed-off-by: Minwoo Im <dn3108@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>