imx: mmc_env: update runtime SD/MMC boot env device

When DM_MMC enabled, the USDHC index in U-Boot is the USDHC port.
To directly return devno, we could avoid add board specific code.

Signed-off-by: Peng Fan <peng.fan@nxp.com>

arch/arm/mach-imx/mmc_env.c

@@ -11,6 +11,9 @@
 
 __weak int board_mmc_get_env_dev(int devno)
 {
+	if (CONFIG_IS_ENABLED(DM_MMC))
+		return devno;
+
 	return CONFIG_SYS_MMC_ENV_DEV;
 }