xilinx: common: Fix static checker warnings

Avoid signed extension for uuid and byte.

Eliminate the below smatch warnings:
board/xilinx/common/board.c:128 xilinx_eeprom_legacy_cleanup()
warn: impossible condition '(byte == 255) => ((-128)-127 == 255)'

board/xilinx/common/board.c:466 board_late_init_xilinx()
warn: argument 3 to %02x specifier has type 'char'
board/xilinx/common/board.c:466 board_late_init_xilinx()
warn: argument 4 to %02x specifier has type 'char'

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Link: https://lore.kernel.org/r/20220926065242.4355-1-venkatesh.abbarapu@amd.com


Signed-off-by: Michal Simek <michal.simek@amd.com>