vm: convert mtdchar mmap to vm_iomap_memory() helper

This is my example conversion of a few existing mmap users.  The mtdchar
case is actually disabled right now (and stays disabled), but I did it
because it showed up on my "git grep", and I was familiar with the code
due to fixing an overflow problem in the code in commit 9c603e53


("mtdchar: fix offset overflow detection").

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>