plat: xilinx: Mask unnecessary bytes of return error code

plat: xilinx: Mask unnecessary bytes of return error code

Versal firmware adds extra error codes along with PM error codes
while sending response to driver. This makes incorrect error
identification at driver side.

To fix this, mask the unnecessary error bytes before sending the
error code to the driver.

Signed-off-by: Ravi Patel <ravi.patel@xilinx.com>
Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Change-Id: I18c2f3bd2d067e91344852c2f0c1bafea0e6eb23