PM / hibernate: use unsigned local variables in swsusp_show_speed()

do_div() needs 'u64' type, or it reports warning. And negative number
is meaningless for "speed", so change all signed to unsigned within
swsusp_show_speed().

The related warning (with allmodconfig for unicore32):

    CC      kernel/power/hibernate.o
  kernel/power/hibernate.c: In function ‘swsusp_show_speed’:
  kernel/power/hibernate.c:237: warning: comparison of distinct pointer types lacks a cast

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
[rjw: Subject]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>