spi-topcliff-pch: Fix probing when DMA mode is used

If during registering SPI master due to SPI device probing a SPI transfer
is issued the DMA buffers are not allocated yet.

This fixes the following oops:
pch_spi 0000:02:0c.1: enabling device (0000 -> 0002)
pch_spi 0000:02:0c.1: master is unqueued, this is deprecated
BUG: unable to handle kernel NULL pointer dereference at   (null)
IP: [<c125aa05>] pch_spi_handle_dma+0x15c/0x6f4
[...]

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Mark Brown <broonie@linaro.org>