drivers/i2c: it51xxx: Fix build assert to check port number instead of inst

Currently the build assert checks the number of I2C instances as
the available I2C target mode, which is incorrect.
This patch updates the build assert to check the I2C port_num instead,
since IT51xxx only supports I2C target mode on ports 0 to 2.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>