rtc: xilinx: Fix calibval variable type

This patch fixes the warnings reported by static code analysis.
Updated calibval variable type to unsigned type from signed.

Signed-off-by: Srinivas Goud <srinivas.goud@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20765c4c27aa92c75426b82fd2815ebef6471492.1570544738.git.michal.simek@xilinx.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>