[MIPS] SB1: Fix CONFIG_SIBYTE_DMA_PAGEOPS build failure.

Fix type mismatch warnings for 64-bit kernel builds which trigger -Werror.
The problem affects only SB-1 kernels with CONFIG_SIBYTE_DMA_PAGEOPS
enabled.

Signed-off-by: Thiemo Seufer <ths@networkno.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>