mtd: move mtd->{get,put}_device functions up

Move the 'get_device()' and 'put_device()' functions up within
'struct mtd_info' to make them be close to other functions.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>