[spifs] Fix warning with GCC 5.3.1

With GCC 5.3.1 (5-2016-q2-update from https://launchpad.net/gcc-arm-embedded)
additional warnings about unused variables cause compilation to fail.

lib/fs/spifs/spifs.c

@@ -152,10 +152,12 @@ static status_t cursor_init(
 
 static uint8_t *cursor_get(cursor_t *cursor)
 {
+#if LK_DEBUGLEVEL > 1
     spifs_t *spifs = cursor->spifs;
 
     uint8_t *page_end = spifs->page + spifs->page_size;
     DEBUG_ASSERT(cursor->data < page_end);
+#endif
 
     return cursor->data;
 }
@@ -336,9 +338,11 @@ static status_t spifs_commit_toc(spifs_t *spifs)
 
     // Sanity check. The cursor should be at the last position in this page
     // at this point.
+#if LK_DEBUGLEVEL > 1
     uint8_t *expected_cursor_location =
         (spifs->page + spifs->page_size) - SPIFS_ENTRY_LENGTH;
     DEBUG_ASSERT(cursor == expected_cursor_location);
+#endif
 
     toc_footer_t *footer = (toc_footer_t *)cursor;
     memset(footer, 0, SPIFS_ENTRY_LENGTH);