mtd: mtdswap: Use module_mtd_blktrans to register driver

Removing some boilerplate by using module_mtd_blktrans instead of calling
register and unregister in the otherwise empty init/exit functions.

Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210213164600.409061-7-zhengdejin5@gmail.com