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>