powerpc/64e: Fix bogus usage of WARN_ONCE()

WARN_ONCE() takes a condition and a format string. We were passing a
constant string as the condition, and the function name as the format
string. It would work, but the message would be just the function name.

Fix it by just using WARN_ONCE() directly instead of if (x) WARN_ONCE().

Noticed-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>