i2c: exynos5: Describe the hardware variant for readability

The driver supports multiple hardware variants of Exynos I2C controller
which differ in FIFO depth, handling of interrupts and bus recovery in
HSI2C_MASTER_ST_LOSE state.

The difference in variant was a single bit set for Exynos7 variants and
implicit lack of this bit for other variants.

Make each variant explicit which also fixes the GCC warning about
documentation:

    drivers/i2c/busses/i2c-exynos5.c:223: warning: Function parameter or member 'hw' not described in 'exynos_hsi2c_variant'

No change in functionality.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>