staging: nvec: specify a platform-device base id

commit 6e3f62f0 (mfd: core: Fix platform-device id generation) modified
the computation of the mfd cell id. Negative numbers forbit the specification
of cell ids as we do. Fix this for now by specifying a base of 0 instead. In
the long run, this may be changed to automatic cell ids (base -2).

Reported-by: Misha Komarovskiy <zombah@gmail.com>
Signed-off-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>