spi/mpc8xxx: fix buffer overrun on large transfers

It fixes an issue when sending-only or receiving-only more than
PAGE_SIZE bytes.

Signed-off-by: christophe leroy <christophe.leroy@c-s.fr>
Acked-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>