usb: gadget: fastboot: add max-download-size variable

Current Android Fastboot seems to use 'max-download-size' instead
of 'downloadsize' variable to indicate the maximum size of sparse
segments.

See function get_target_sparse_limit() in file fastboot/fastboot.c
in the AOSP:
	 https://android.googlesource.com/platform/system/core/+/master

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>

drivers/usb/gadget/f_fastboot.c

@@ -351,7 +351,8 @@ static void cb_getvar(struct usb_ep *ep, struct usb_request *req)
 		strncat(response, FASTBOOT_VERSION, chars_left);
 	} else if (!strcmp_l1("bootloader-version", cmd)) {
 		strncat(response, U_BOOT_VERSION, chars_left);
-	} else if (!strcmp_l1("downloadsize", cmd)) {
+	} else if (!strcmp_l1("downloadsize", cmd) ||
+		!strcmp_l1("max-download-size", cmd)) {
 		char str_num[12];
 
 		sprintf(str_num, "%08x", CONFIG_USB_FASTBOOT_BUF_SIZE);