media: cadence: csi2rx: Fix uninitialized used_vc variable

commit 215fa0c6d1e6de9e1a22105bbe728e1de614292d from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git

Initialize the used_vc variable to 0 to prevent lkp warning for
cases where source provides frame descriptor that does not populate
virtual channel for every stream.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406011359.qVrQBfFU-lkp@intel.com/
Signed-off-by: Jai Luthra <j-luthra@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>