um: Clean up alarm IRQ chip name

We don't use "SIGVTALRM", it's just SIGALRM. Clean up the naming.
While at it, fix the comment's grammar.

Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>