drivers/fsi: Add master unscan

Allow a master to undo a previous scan.  Should a master scan a bus
twice it will need to ensure it doesn't double register any
previously detected device.

Signed-off-by: Christopher Bostic <cbostic@linux.vnet.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
----
v7 - Unscan when unregistering master
   - Remove leading '__'s from function names
   - Return fail state for sysfs rescan file
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>