spi: mpc8xx: Fix transfert when input or output buffer is NULL

spi: mpc8xx: Fix transfert when input or output buffer is NULL

xfer ops can be passed a NULL input or output buffer. At the
time being the driver ignores it and overwrites memory at 0.

Define a dummy buffer and use it when either input or output
buffer is NULL. Bail out when both are NULL as it shouldn't.

Also increase MAX_BUFFER len to 32k as the current is pretty
low.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>