i2c: imx_lpi2c: Replace hard-coded bus speed value with bus->speed_hz

Instead of using the hard-coded bus speed value I2C_SPEED_STANDARD_RATE,
use the actual configured bus speed. This way the bus speed doesn't
change suddenly after calling the imx_lpi2c_probe_chip() function for
example.

Signed-off-by: Fedor Ross <fedor.ross@ifm.com>