frv: kill used but uninitialized variable

Commit 6afe1a1fe8ff ("PM: Remove legacy PM") removed the initialization
of retval, causing:

  arch/frv/kernel/pm.c: In function 'sysctl_pm_do_suspend':
  arch/frv/kernel/pm.c:165:5: warning: 'retval' may be used uninitialized in this function [-Wuninitialized]

Remove the variable completely to fix this, and convert to a proper
switch (...) { ... } construct to improve readability.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>