usb-storage: reorganize target-specific code

Now that usb-storage has a target_alloc() routine, this patch (as1508)
moves some existing target-specific code out of the slave_alloc()
routine to where it really belongs.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>