soc:qcom:spcom: fix ion buffer unlock on rpmsg dev remove

In case of SSR event all spss channels are disconnected. This causes each
rpmsg device remove. The channel context keep track of locked ion buffers
that were shared with SP. On SSR event only "sp_kernel" channel ion buffers
must be unlocked (because userspace will not do it by design).

This change instead of unlocking buffers for all channels, unlocks only
"sp_kernel" related ion buffers.

Change-Id: I2e677a6f301e09922a0c8238e5aaf3b65bf99176
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>