staging: vt6655: Fix unused function warning

Sparse reports that MimeThread is not used. Actually, it can be used
if THREAD is defined. By enclosing the MimeThread function into the
same #ifdef as the caller of MimeThread, this fixes the sparse
warnings.

Signed-off-by: Guillaume Clement <gclement@baobob.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>