staging: mt7621-dma: fix potentially dereferencing uninitialized 'tx_desc'

Function 'mtk_hsdma_start_transfer' uses 'tx_desc' pointer which can be
dereferenced before it is initializated. Initializate pointer before
avoiding the problem.

Fixes: 0853c7a5 ("staging: mt7621-dma: ralink: add rt2880 dma engine")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>