hw/sd/sdcard: Assert if accessing an illegal group

We can not have more group than 'wpgrps_size'.
Assert if we are accessing a group above this limit.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20201015063824.212980-7-f4bug@amsat.org>