mtd: silence some uninitialized variable warnings

The "tmp_retlen" variable can be uninitialized if action() fails.  It's
harmless except for the static checker warning.  I have moved the error
handling earlier to fix it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>