media: ttusb-dec: Remove pci_zalloc_coherent() abuse

Switch to a plain kzalloc() instead of pci_zalloc_coherent() to allocate
memory for the USB DMA.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>