media: em28xx: don't use coherent buffer for DMA transfers

While coherent memory is cheap on x86, it may cause performance
impacts on other archs. As we don't have any good reason to
use it, let's change the logic by allocating memory via kmalloc()
and letting the USB core to do the DMA mapping and memory free
for us.

While here, also fixes an issue that it was not de-allocating
memories if something gets wrong during memory block
allocation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>