PM / sleep: Correct whitespace errors in <linux/pm.h>

rjw> Why exactly are they errors?
Geert> checkpatch.pl says: "WARNING: please, no space before tabs",
       Vim (with "let c_space_errors=1") shows them in red.
Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>