cramfs: better MTD dependency expression

Commit b9f5fb18 ("cramfs: fix MTD dependency") did what it says.

Since commit 9059a349 ("kconfig: fix relational operators for bool
and tristate symbols") it is possible to do it slightly better though.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>