drivers/char/ipmi/ipmi_si_intf.c: fix cleanup_one_si section mismatch

commit d2478521 ("char/ipmi: fix OOPS caused by
pnp_unregister_driver on unregistered driver") introduced a section
mismatch by calling __exit cleanup_ipmi_si from __devinit init_ipmi_si.

Remove __exit annotation from cleanup_ipmi_si.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>