sparc: leon: Fix race condition between leon_cycles_offset and timer_interrupt

This makes sure that leon_cycles_offset takes the pending bit into
account and that leon_clear_clock_irq clears the pending bit. Otherwise,
if leon_cycles_offset is executed after the timer has wrapped but before
timer_interrupt has increased timer_cs_internal_counter, time can be
perceived to go backwards.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>