Blackfin: fix timer_init()/timer_reset()

The timer_init() function was not using the right csync instruction, nor
was it doing it right after disabling the core timer.

The timer_reset() function would reset the timestamp, but not the actual
timer, so there was a common edge case where get_timer() return a jump of
one timestamp (couple milliseconds) right after resetting.  This caused
many functions to improperly timeout right away.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>