mtip32xx: restrict variables visible in current code module

The modified variables are only used in the file mtip32xx.c.
As such, the static keyword is inserted to define that object
to be only visible to the current code module during compilation.

Signed-off-by: Zhu Yanjun <zyjzyj2000@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>