mx6: Use imx6_src_get_boot_mode() to check boot device

Use imx6_src_get_boot_mode() instead of manually reading SBMR1.  The
existing function has proper handling for software overrides of the
bootdevice which can happen, for example, when booting from an alternate
source using `bmode`.

Signed-off-by: Harald Seiler <hws@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>

arch/arm/mach-imx/mx6/soc.c

@@ -498,8 +498,7 @@ __weak int board_mmc_get_env_dev(int devno)
 
 static int mmc_get_boot_dev(void)
 {
-	struct src *src_regs = (struct src *)SRC_BASE_ADDR;
-	u32 soc_sbmr = readl(&src_regs->sbmr1);
+	u32 soc_sbmr = imx6_src_get_boot_mode();
 	u32 bootsel;
 	int devno;