mtd: fix 'Flash device refused suspend due to active operation' message

While debugging on SA11x0, the following message was observed:

"Flash device refused suspend due to active operation (state 20)"

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>