rtc: ds1685: use threaded interrupt

Handling of extended interrupts (kickstart, wake-up, ram-clear) was
moved off to a work queue, but the interrupts aren't acknowledged
in the interrupt handler. This leads to a deadlock, if driver
is used with interrupts. To fix this we use a threaded interrupt, get rid
of the work queue and do locking with just the rtc mutex lock.

Fixes: aaaf5fbf ("rtc: add driver for DS1685 family of real time clocks")
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>