mmc: sdhci-of-aspeed: Fix kunit-related build error

mmc: sdhci-of-aspeed: Fix kunit-related build error

Randy found that with the following Kconfig settings we have duplicate
definitions (e.g. __inittest()) in sdhci-of-aspeed due to competing
module_init()/module_exit() calls from kunit and driver the itself.

```
CONFIG_MMC_SDHCI_OF_ASPEED=m
CONFIG_MMC_SDHCI_OF_ASPEED_TEST=y
```

Conditionally open-code the kunit initialisation to avoid the error.

Fixes: 7efa02a981d6 ("mmc: sdhci-of-aspeed: Add KUnit tests for phase calculations")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/r/20210122114852.3790565-1-andrew@aj.id.au
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>