drivers/rtc/rtc-bq32k.c: fix register value

Fix register value in bq32000 trickle charging.

Mike reported that I'm using wrong value in one trickle-charging case,
and after checking docs, I must admit he's right.

Signed-off-by: Pavel Machek <pavel@denx.de>
Reported-by: Mike Bremford <mike@bfo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>