staging: nvec: convert to use GPIO descriptors

Use GPIO descriptors instead of relying on the old method.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>