md: Fix raid1->raid0 takeover

Takeover raid1->raid0 not succeded. Kernel message is shown:
"md/raid0:md126: too few disks (1 of 2) - aborting!"

Problem was that we weren't updating ->raid_disks for that
takeover, unlike all the others.

Signed-off-by: Krzysztof Wojcik <krzysztof.wojcik@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>