mtd: blkdevs: do not for get to get MTD devices

Nowadays MTD devices have to be "get" before they can be
used. This has to be done with 'put_mtd_device()'. The
'blktrans_open()' function did not do this and instead
used 'try_module_get()'. Fixe this.

Since 'put_mtd_device()' already gets the module, extra
'try_module_get()' is not needed.

This fixes oops when one tries to use mtdbloc on tope of
gluebi.

Reported-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>