zephyr: Fix serial recovery compilation warnings

Missing const on pointers to device structures caused compilation
warnings when compiling bootloader with serial recovery enabled.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index 0efc5a8..d05939e 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -324,7 +324,7 @@
 
 #ifdef CONFIG_MCUBOOT_SERIAL
 
-    struct device *detect_port;
+    struct device const *detect_port;
     uint32_t detect_value = !CONFIG_BOOT_SERIAL_DETECT_PIN_VAL;
 
     detect_port = device_get_binding(CONFIG_BOOT_SERIAL_DETECT_PORT);