powerpc/powernv: Remove "opal" prefix from pr_xxx()s

In commit c8742f85 "powerpc/powernv: Expose OPAL firmware symbol
map" I added pr_fmt() to opal.c. This left some existing pr_xxx()s with
duplicate "opal" prefixes, eg:

    opal: opal: Found 0 interrupts reserved for OPAL

Fix them all up. Also make the "Not not found" message a bit more
verbose.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>