ASoC: wcd-spi: cleanup to remove unused SPI APIs

spi data read and write are handled by WDSP_EVENT_GETOPS
and can be removed from being exported, rather make them
static.

CRs-Fixed: 2068106
Change-Id: I815f0e7c2ac02236e23c7d1b98929dad93e48d6d
Signed-off-by: Karthikeyan Mani <kmani@codeaurora.org>