ASoC: wcd: compute impedance value for headset

While performing impedance detection, compute
the values of impedance found in milliohms.

CRs-Fixed: 698474
Change-Id: I0570db9a7a5df32a985d1654a913234aad74a558
Signed-off-by: Simmi Pateriya <simmip@codeaurora.org>