drivers/rtc/interface.c: return -EBUSY, not -EACCES when device is busy

If rtc->irq_task is non-NULL and task is NULL, they always
rtc_irq_set_freq(), whenever err is set to -EBUSY it will then immediately
be set to -EACCES, misleading the caller as to the underlying problem.

Signed-off-by: Chris Brand <chris.brand@broadcom.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>