[MTD] [NOR] fix ctrl-alt-del can't reboot for intel flash bug

When we press ctrl-alt-del,kernel_restart_prepare will invoke
cfi_intelext_reboot which will set flash to read array mode, but later
when device_shutdown is invoked which may put current work queue to
sleep and other process may be scheduled to running and programming
flash in not FL_READY mode again. So we can't boot up if this flash is
used for bootloader.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>