i2c:multibus:fix: Correct I2C_MULTI_BUS value when support for many buses is enabled

The I2C_MULTI_BUS needs to be defined for correct I2C operation with
many software emulated I2C controllers.

This fix restores correct value of the I2C_MULTI_BUS changed by following
commit:

SHA1: 385c9ef5
i2c: add i2c_core and prepare for new multibus support

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Heiko Schocher <hs@denx.de>