ASoC: wcd9xxx: fix out of bound access

Somtimes index can be more than array size of imped_index.
which can lead to out of bound access. Fix this by checking
for valid value of index.

CRs-Fixed: 2000364
Change-Id: Ie571992f7a0b8156b02e09bb3adb0f9c9a370df7
Signed-off-by: Yeleswarapu Nagaradhesh <nagaradh@codeaurora.org>