Blackfin RTC driver: convert sync wait to use the irq write complete …

…notice

 - thus clearing out the need for spin locks
 - add a small optimization for reading of the rtc field

Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Acked-by: Alessandro Zummo <alessandro.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>