i2c: mvtwsi: sunxi: update macro

While currently none of the newer Allwinner SoCs currently has I2C
support implemented in U-Boot, this will change soon. mvtwsi driver is
good as it is for them except one macro. Update it to be ready once I2C
support lands for those SoCs.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>