powerpc/eeh_cache: fix a W=1 kernel-doc warning

The opening comment mark "/**" is reserved for kernel-doc comments, so
it will generate a warning with "make W=1".

arch/powerpc/kernel/eeh_cache.c:37: warning: cannot understand function
prototype: 'struct pci_io_addr_range

Since this is not a kernel-doc for the struct below, but rather an
overview of this source eeh_cache.c, just use the free-form comments
kernel-doc syntax instead.

Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>