Staging: ramzswap: Return proper error code on device create failure

Currently, we return 0 if create_device() fails and 1 otherwise.
Now, proper error code is returned from create_device() and the
same is propagated as module error code from ramzswap_init().

Also added some cleanups for ramzswap_init(), improving function
structure.

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>