staging: nvec: Return error in PS2 driver if write fails

Return the return value of nvec_write_async() in the methods
returning an int.

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>