i2c: imx_lpi2c: Fix misuse the IS_ENABLED for DM clock

The IS_ENABLED, which does not consider SPL build, should be replaced
by CONFIG_IS_ENABLED.
For the case that we only enable DM CLK for u-boot but not in SPL, the
IS_ENABLED(CONFIG_CLK) still returns true, then cause clock failure.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>