i2c_posix: fix use of wrong device path

previously, get_devname() was used as the I2C device path, but on NuttX,
get_devname() is the device file which the driver creates. This patch
changes it, so the sematics are the same as on NuttX: both now use _bus
to decide to which I2C bus device to talk to.

I did not see any other use-cases than the led on ocpoc.