board: asus: transformer-t30: enable I2C_MUX only for TF700T

Of all T30 transformers, only the TF700T uses GPIO i2c muxing
for one of the i2c lines and needs this driver to properly work.
Disable this configuration for all transformers except tf700t
in their fragments.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>