hw/microblaze: Replace drive_get_next() by drive_get()

hw/microblaze: Replace drive_get_next() by drive_get()

drive_get_next() is basically a bad idea.  It returns the "next" block
backend of a certain interface type.  "Next" means bus=0,unit=N, where
subsequent calls count N up from zero, per interface type.

This lets you define unit numbers implicitly by execution order.  If the
order changes, or new calls appear "in the middle", unit numbers change.
ABI break.  Hard to spot in review.

Machine "petalogix-ml605" connects backends with drive_get_next() in a
counting loop.  Change it to use drive_get() directly.  This makes the
unit numbers explicit in the code.

Cc: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20211117163409.3587705-10-armbru@redhat.com>
Acked-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>