fastboot: preserve partition_type if wants_wipe=true

originally if wants_wipe=true, program will erase and format partitions,
but after fb->Erase(), partition type should be raw(all 0xFFs),
then following fb_perform_format() will miss its original partition_type
Now we call fb_perform_format() with original fs type to keep the
partition consistent after wiping.

Change-Id: Ic778850588d5dd6fee23169d20c26bcbaa510627