[S390] dasd: fix race in dasd timer handling

In dasd_device_set_timer and dasd_block_set_timer we interpret the
return value of mod_timer in a wrong way. If the timer expires in
the small window between our check of timer_pending and the call to
mod_timer, then the timer will be set, mod_timer returns zero and
we will call add_timer for a timer that is already pending.
As del_timer and mod_timer do all the necessary checking themselves,
we can simplify our code and remove the race a the same time.

Signed-off-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>