init/Kconfig: Hide printk log config if CONFIG_PRINTK=n

commit 361e9dfbaae84b0b246ed18d1ab7c82a1a41b53e upstream.

The buffers sized by CONFIG_LOG_BUF_SHIFT and
CONFIG_LOG_CPU_MAX_BUF_SHIFT do not exist if CONFIG_PRINTK=n, so don't
ask about their size at all.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
[bwh: Backported to 3.2: drop change to CONFIG_LOG_CPU_MAX_BUF_SHIFT]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>