media: venus: vdec: Fix decoder cmd STOP issue

Fixes an issue when issuing a stop command to the controller, negating the
following firmware error.

"SFR message from FW: Exception: TID = Unknown IP = 0x3b7dc FA = 0x0
 cause = 0x6"

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>