mmc: zynq_sdhci: Resolve uninitialized return value

mmc: zynq_sdhci: Resolve uninitialized return value

set_phase() functions are not modifying the ret value and returning
the same uninitialized ret, return 0 instead.

Keep the return type as int to return errors when the tapdelay's are
set via xilinx_pm_request() in future.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>