tests: lib: cmsis_nn: Initialise bias dimensions for depthwise convolve

This commit adds an initialisation for the `bias_dims` variable, which
is given as an input to the `arm_depthwise_conv_s8` function.

Note that the `bias_dims` parameter is currently unused by the function
implementation.

This fixes the following warning reported by the GCC 12:

  error: 'bias_dims' may be used uninitialized
  Werror=maybe-uninitialized]

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>