[sw/silicon_creator] Clean up flash disable in shutdown

The flash disable register was originally rw1c but is now rw0c.
Writing 1 to the register does actually work because the reset value
of the register is 5 but writing 0 to the register is cleaner.

Signed-off-by: Michael Munday <mike.munday@lowrisc.org>
diff --git a/sw/device/silicon_creator/lib/shutdown.c b/sw/device/silicon_creator/lib/shutdown.c
index dac24e9..b932df8 100644
--- a/sw/device/silicon_creator/lib/shutdown.c
+++ b/sw/device/silicon_creator/lib/shutdown.c
@@ -294,7 +294,8 @@
 
 SHUTDOWN_FUNC(NO_MODIFIERS, shutdown_flash_kill(void)) {
   enum { kBase = TOP_EARLGREY_FLASH_CTRL_CORE_BASE_ADDR };
-  abs_mmio_write32(kBase + FLASH_CTRL_DIS_REG_OFFSET, 1);
+  // Setting DIS (rw0c) to a value other than 5 will disable flash permanently.
+  abs_mmio_write32(kBase + FLASH_CTRL_DIS_REG_OFFSET, 0);
 }
 
 SHUTDOWN_FUNC(noreturn, shutdown_hang(void)) {