xilinx: board: Fix xilinx_eeprom_legacy_cleanup()

When ethernet mac address contains 0x20 or 0xff MAC address is changed and
bytes are converted to zeros. That's why fix decoding algorithm to ignore
fields where MAC address is stored and all non printable chars (including
space) are zeroed.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/2802cf1086b14c181356810006fe886f950a36f3.1674573561.git.michal.simek@amd.com