mmc: sdhci-of-at91: fix CALCR register being rewritten

mmc: sdhci-of-at91: fix CALCR register being rewritten

When enabling calibration at reset, the CALCR register was completely
rewritten. This may cause certain bits being deleted unintentedly.
Fix by issuing a read-modify-write operation.

Fixes: 727d836a375a ("mmc: sdhci-of-at91: add DT property to enable calibration on full reset")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20200527105659.142560-1-eugen.hristev@microchip.com
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>