msm: vidc: Fix scope of "mode" variable at try_set_ctrl function

The variable was declared inside a switch case
statement but it is used outside the statement
causing random errors if use.

Change-Id: Iffe2c182f8057720d8e9db9e73fba1512a1c88f1
Signed-off-by: Jorge Solano Altamirano <jsolano@codeaurora.org>