net: iosm: Prevent underflow in ipc_chnl_cfg_get()

The bounds check on "index" doesn't catch negative values.  Using
ARRAY_SIZE() directly is more readable and more robust because it prevents
negative values for "index".  Fortunately we only pass valid values to
ipc_chnl_cfg_get() so this patch does not affect runtime.

Reported-by: Solomon Ucko <solly.ucko@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: M Chetan Kumar <m.chetan.kumar@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>