ASoC: spear: correct the check for NULL dma_buffer pointer

The if condition
	if (!buf && !buf->area)

checks if the buf pointer is NULL and then dereferences it again to
check if the buffer area is NULL, resulting in possible NULL
dereference.

Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>