HD300: Fix crash in bootloader introduced by 36281c4

Uninitialized struct scroll which is used to pass state between
scrollstrip ISR and button_read_device() can bomb out whole
button subsytem.

Change-Id: I3b415c22cfee4181b2132cddaeff68797c7cc0ea

firmware/target/coldfire/mpio/hd300/button-hd300.c

@@ -38,7 +38,10 @@ static volatile struct scroll_state_t {
     signed char dir;
     long timeout;
     bool rel;
-} scroll;
+} scroll = { .dir = BUTTON_UP,
+             .timeout = SLIDER_REL_TIMEOUT,
+             .rel = false,
+           };
 
 static inline void disable_scrollstrip_interrupts(void)
 {