m68knommu: fix broken ColdFire slice timer read_clk() code

There is a race on reading the ColdFire slice timer current count and the
total clock count so far. Interrupts are off, and we may have just missed
getting a new timer wrap event interrupt. Check for this and adjust the
cycle count and current read count accordingly.

Also the slice timer counts down from the terminal count. So in read_clk()
we need take the current clock count away from the terminal count.

Reported-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>