xilinx_timer: Fixed deadlock issue

The timer was deadlocking when the interval was set too low. It would cause a
flood of timer events and the CPU would halt indefinately. This is a known issue
and theres a generic workaround in place in ptimer on ptimer_set_limit(),
however the Xilinx timer uses ptimer_set_count() instead of set_limit. Changed
the call to set_count() to an equivalent call of set_limit() instead, which
brings the workaround into play.

Signed-off-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
