rtc: zynqmp: initialize fract_tick

fract_tick is used uninitialized when fract_offset is 0

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20220727100018.3301470-1-alexandre.belloni@bootlin.com