ASoC: wcd-spi: Remove debugfs node on unbind

spi creates a debugfs entry at component bind
which has to be remove at component unbind so
that at consecutive load and unload of module
debugfs entry is added and removed without
conflicts.

CRs-fixed: 2092930
Change-Id: Ic0f4bcb507fc4b2e6f9df2e08da588cde2a78a2e
Signed-off-by: Karthikeyan Mani <kmani@codeaurora.org>