ramoops: fix use of rounddown_pow_of_two()

commit fdb5950754eb3dedb9fea7c8828d3e51d9dbc3f7 upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>