sfc: Fix oops in register dump after mapping change

Commit 747df225 ('sfc: Always map MCDI
shared memory as uncacheable') introduced a separate mapping for the
MCDI shared memory (MC_TREG_SMEM).  This means we can no longer easily
include it in the register dump.  Since it is not particularly useful
in debugging, substitute a recognisable dummy value.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/sfc/nic.c

@@ -1937,6 +1937,13 @@ void efx_nic_get_regs(struct efx_nic *efx, void *buf)
 
 		size = min_t(size_t, table->step, 16);
 
+		if (table->offset >= efx->type->mem_map_size) {
+			/* No longer mapped; return dummy data */
+			memcpy(buf, "\xde\xc0\xad\xde", 4);
+			buf += table->rows * size;
+			continue;
+		}
+
 		for (i = 0; i < table->rows; i++) {
 			switch (table->step) {
 			case 4: /* 32-bit register or SRAM */