mtd-rw: fix build failure with kernel 5.10

When building mtd-rw for a target based on Kernel 5.10 which has
CONFIG_MTD disabled the build fails with

ERROR: "put_mtd_device" [../mtd-rw.ko] undefined!
ERROR: "get_mtd_device" [../mtd-rw.ko] undefined!

Omit building the package for such a target.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit c6b3d949)