ubi: gluebi: Fix double refcounting

There is no need to call get/put on the module
reference in gluebi_get/put_device() callbacks.
Since mtd->owner is the gluebi module itself
mtdcore.c will take care of proper refcounting
in __get/put_mtd_device() before executing the
callbacks.

Signed-off-by: Richard Weinberger <richard@nod.at>