hw/sd/sdcard: Document out-of-range addresses for SEND_WRITE_PROT

Per the 'Physical Layer Simplified Specification Version 3.01',
Table 4-22: 'Block Oriented Write Protection Commands'

  SEND_WRITE_PROT (CMD30)

  If the card provides write protection features, this command asks
  the card to send the status of the write protection bits [1].

  [1] 32 write protection bits (representing 32 write protect groups
  starting at the specified address) [...]
  The last (least significant) bit of the protection bits corresponds
  to the first addressed group. If the addresses of the last groups
  are outside the valid range, then the corresponding write protection
  bits shall be set to 0.

Split the if() statement (without changing the behaviour of the code)
to better position the description comment.

Reviewed-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210802235524.3417739-2-f4bug@amsat.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Alexander Bulekov <alxndr@bu.edu>

hw/sd/sd.c

@@ -822,7 +822,14 @@ static uint32_t sd_wpbits(SDState *sd, uint64_t addr)
 
     for (i = 0; i < 32; i++, wpnum++, addr += WPGROUP_SIZE) {
         assert(wpnum < sd->wpgrps_size);
-        if (addr < sd->size && test_bit(wpnum, sd->wp_groups)) {
+        if (addr >= sd->size) {
+            /*
+             * If the addresses of the last groups are outside the valid range,
+             * then the corresponding write protection bits shall be set to 0.
+             */
+            continue;
+        }
+        if (test_bit(wpnum, sd->wp_groups)) {
             ret |= (1 << i);
         }
     }