Staging: nvec: Modify the nvec_write_sync method to return the error code

Modify the nvec_write_sync function to return the error code
instead of the received message.

The received message is now returned as an output parameter.

Signed-off-by: Tomás Tormo <tomas.tormo@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>