i2c: exynos5: Preserve high speed master code

When the driver starts to send a message with the MASTER_ID field
set (high speed), the whole I2C_ADDR register is overwritten including
MASTER_ID as the SLV_ADDR_MAS field is set.

This patch preserves already written fields in I2C_ADDR when writing
SLV_ADDR_MAS.

Fixes: 8a73cd4c ("i2c: exynos5: add High Speed I2C controller driver")
Signed-off-by: Mårten Lindahl <martenli@axis.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>