staging: nvec: Have nvec_write_async() return -ENOMEM on OOM

Change nvec_write_async() to return an integer, 0 by default,
a negative error on failure. Change nvec_write_sync() to
check the return value and abort if it is negative.

Signed-off-by: Julian Andres Klode <jak@jak-linux.org>
Acked-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>