staging: spectra: asynchronous init

The spectra nand driver takes quite some time to initialize because
it needs to scan the whole nand disk to find the latest block table.
This patch initializes the spectra nand driver asynchronously so that
other things in the kernel can initialize in parallel to the scanning
operation.

Signed-off-by: Yong Wang <yong.y.wang@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>