mmc: sdhci-of-at91: fix quirk2 overwrite

The quirks2 are parsed and set (e.g. from DT) before the quirk for broken
HS200 is set in the driver.
The driver needs to enable just this flag, not rewrite the whole quirk set.

Fixes: 7871aa60 ("mmc: sdhci-of-at91: add quirk for broken HS200")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>