hw/sd/sdcard: Introduce the INVALID_ADDRESS definition

'0' is used as a value to indicate an invalid (or unset)
address. Use a definition instead of a magic value.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20201015063824.212980-3-f4bug@amsat.org>