[BUGFIX] Validation problem in forgot and change password form

Label for attribute is invalid for XHTML. Changing it to a hard coded
value solves the problem. In case custom styling or Javascript breaks
the whole template can be replaced.

Change-Id: I41008125706f51104ff5ddf16786f0d7a71cf2af
Fixes: #18901
Releases: 6.0, 4.7
Reviewed-on: http://review.typo3.org/7514
Reviewed-by: Oliver Klee
Reviewed-by: Sebastian Michaelsen
Tested-by: Sebastian Michaelsen
Reviewed-by: Andy Grunwald
Tested-by: Andy Grunwald
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn