s3cmci: fix sparse errors from non-exported functions

Fix the following sparse errors by making the functions
static and fixing the check for host->base.

598:6: warning: symbol 's3cmci_dma_done_callback' was not declared. Should it be static?
744:6: warning: symbol 's3cmci_dma_setup' was not declared. Should it be static?
1209:20: warning: Using plain integer as NULL pointer

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>