nouveau: fix type warnings: void* arith, un/signed

nouveau_notifier.c had two places where void* was used in arithmetic,
fixed by using char*.

nouveau_dma_wait(), nouveau_notifier_wait_status() and
nouveau_resource_alloc() had signed/unsigned comparison warnings, fixed
by changing the function parameter into an unsigned type.

Signed-off-by: Pekka Paalanen <pq@iki.fi>