nitrogen8m_som: don't return to ROM if CONFIG_SPL_USB_SDP_SUPPORT defined

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>

board/boundary/nitrogen8m_som/spl.c

@@ -188,11 +188,13 @@ void spl_board_init(void)
 	/* DDR initialization */
 	spl_dram_init();
 
+#ifndef CONFIG_SPL_USB_SDP_SUPPORT
 	/* Serial download mode */
 	if (is_usb_boot()) {
 		puts("Back to ROM, SDP\n");
 		restore_boot_params();
 	}
+#endif
 	puts("Normal Boot\n");
 }