drivers/rtc/rtc-ds1307.c: change variable type to bool

The variable want_irq is only assigned the values true and false.
Change its type to bool.

The simplified semantic patch that find this problem is as
follows (http://coccinelle.lip6.fr/

):

@exists@
type T;
identifier b;
@@
- T
+ bool
  b = ...;
  ... when any
  b = \(true\|false\)

Signed-off-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>