mailbox: Convert timers to use timer_setup()

In preparation for unconditionally passing the struct timer_list pointer to
all timer callbacks, switch to using the new timer_setup() and from_timer()
to pass the timer pointer explicitly.

Cc: Ley Foon Tan <lftan@altera.com>
Cc: Jassi Brar <jassisinghbrar@gmail.com>
Cc: nios2-dev@lists.rocketboards.org
Signed-off-by: Kees Cook <keescook@chromium.org>