powerpc/powernv: Properly fix LPC debugfs endianness

Endian is hard, especially when I designed a stupid FW interface, and
I should know better... oh well, this is attempt #2 at fixing this
properly. This time it seems to work with all access sizes and I
can run my flashing tool (which exercises all sort of access sizes
and types to access the SPI controller in the BMC) just fine.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: stable@vger.kernel.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>