USB: musb: don't dereference NULL tusb_dma in dma_controller_destroy()

Test whether tusb_dma is not NULL before dereferencing

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>