aspeed/smc: Fix possible integer overflow

aspeed/smc: Fix possible integer overflow

Coverity reports a possible integer overflow because routine
aspeeed_smc_hclk_divisor() has a codepath returning 0, which could
lead to an integer overflow when computing variable 'hclk_shift' in
the caller aspeed_smc_dma_calibration().

The value passed to aspeed_smc_hclk_divisor() is always between 0 and
15 and, in this case, there is always a matching hclk divisor. Remove
the return 0 and use g_assert_not_reached() instead.

Fixes: Coverity CID 1547822
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>