fastboot: Make libata initialization even more async

As suggested by Linus: Don't do the libata init in 2 separate
steps with a global sync inbetween, but do it as one async step,
with a local sync before registering the device.

This cuts the boottime on my machine with 2 sata controllers down
significantly, and it seems to work. Would be nice if the libata
folks take a good look at this patch though..

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>