zynq: timer: Fix wrong timer calculation

Fix wrong timer calculation in get_timer_masked incase of
overflow.
This fixes the issue of getting wrong time from get_timer()
calls.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>