ata_piix: Add MODULE_PARM_DESC to prefer_ms_hyperv

In reference to the commit cd006086fa5d91414d8ff9ff2b78fbb593878e3c
"ata_piix: defer disks to the Hyper-V drivers by default",
this trivial patch adds a description to prefer_ms_hyperv.

[rvrbovsk@redhat.com: MODULE_PARM_DESC() string formatting modified]

Signed-off-by: Andrew Brownfield <abrownfi@redhat.com>
Signed-off-by: Radomir Vrbovsky <rvrbovsk@redhat.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>