soc: qcom: glink_spi_xprt: Change to spi function pointer interface

Use function pointers provided by the wcd spi driver instead of
using the exposed read and write functions. This change helps remove
the static dependency on the wcd spi driver.

CRs-Fixed: 2068106
Change-Id: I61121dd83079767f73a412e40f9f4ac7198fb210
Signed-off-by: Chris Lew <clew@codeaurora.org>