Blackfin: reset watchdog in udelay()

All arches apparently should reset the watchdog in their udelay loop as
noted on the mailing list recently:

  > A comment in flash_status_check() suggests that udelay() is
  > expected to reset the watchdog, but I can't find any architecture
  > where it does.

  If this is missing in other architectures, it should be fixed at the
  root cause, i. e. in udelay() or in the respective support routines.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>