arm: zynq: timer: Remove unnecessary register write

Acknowedging an interrupt requires to read the interrupt register
only. The write was only required to work around a bug in
the QEMU implementation of the TTC, which is fixed.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Tested-by: Josh Cartwright <josh.cartwright@ni.com>