am335x_evm: Fix DFU for eMMC

Use dfu_alt_info_emmc variable from include/environment/ti/dfu.h file.
It was probably overlooked when extracting DFU variables to mentioned
file.

This patch fixes DFU on BeagleBone Black, so that we can use commands
like ones below to upgrade various images on eMMC:

    => setenv dfu_alt_info $dfu_alt_info_emmc
    => dfu 0 mmc 1

    $ dfu-util -D MLO -a MLO.raw
    $ dfu-util -D u-boot.img -a u-boot.img.raw

Without this patch, the  user is forced to assign the value to
dfu_alt_info_emmc manually, which contradicts with instructions [1].

[1] http://processors.wiki.ti.com/index.php/Linux_Core_U-Boot_User%27s_Guide

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>

include/configs/am335x_evm.h

@@ -255,7 +255,7 @@
 /* USB Device Firmware Update support */
 #ifndef CONFIG_SPL_BUILD
 #define DFUARGS \
-	"dfu_alt_info_emmc=rawemmc raw 0 3751936\0" \
+	DFU_ALT_INFO_EMMC \
 	DFU_ALT_INFO_MMC \
 	DFU_ALT_INFO_RAM \
 	DFU_ALT_INFO_NAND