mtip32xx: mark symbols static where possible

We get 1 warning when biuld kernel with W=1:
drivers/block/mtip32xx/mtip32xx.c:3689:6: warning: no previous prototype for
 'mtip_block_release' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is declared
and don't need a declaration, but can be made static.
so this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Jens Axboe <axboe@fb.com>