ASoC: dsp: Decrement Backend only when state is

SND_SOC_DSP_LINK_STATE_FREE

During device derouting of a single stream to multiple devices,
when one backend is freed, second backend instance count is
wrongly removed since state condition is checked after the
instance removal.
Fix the issue by checking the backend state, before removal.
Remove the backend only on state SND_SOC_DSP_LINK_STATE_FREE.

CRs-fixed: 313625
Change-Id: Ic929f29cae2cd23fc45b4b145b270054d4e9b49a
Signed-off-by: Jayasena Sangaraboina <jsanga@codeaurora.org>