ramoops: fix use of rounddown_pow_of_two()

The return value of rounddown_pow_of_two wasn't evaluated, so the
operation was a no-op.

Signed-off-by: Marco Stornelli <marco.stornelli@gmail.com>
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>