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.

posix-configs/ocpoc/px4.config

@@ -11,7 +11,7 @@ param set SYS_MC_EST_GROUP 2
 df_mpu9250_wrapper start_without_mag -R 10
 df_hmc5883_wrapper start
 df_ms5611_wrapper start
-rgbled start -b /dev/i2c-1
+rgbled start -b 1
 ocpoc_adc start
 gps start -d /dev/ttyS5 -s
 sensors start