media: dvb-ttusb-budget: don't use stack for USB transfers

As reported by smatch:

    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:311 ttusb_boot_dsp() error: doing dma on the stack (b)
    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:321 ttusb_boot_dsp() error: doing dma on the stack (b)
    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:330 ttusb_boot_dsp() error: doing dma on the stack (b)
    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:351 ttusb_set_channel() error: doing dma on the stack (b)
    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:361 ttusb_del_channel() error: doing dma on the stack (b)
    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:412 ttusb_init_controller() error: doing dma on the stack (b0)
    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:416 ttusb_init_controller() error: doing dma on the stack (b1)
    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:422 ttusb_init_controller() error: doing dma on the stack (b2)
    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:425 ttusb_init_controller() error: doing dma on the stack (b3)
    drivers/media/usb/ttusb-budget/dvb-ttusb-budget.c:430 ttusb_init_controller() error: doing dma on the stack (get_version)

This driver still uses the USB stack for DMA transfers,
which is broken for a long time. I almost dropped this driver,
as there's a high chance that nobody is using it with upstream
Kernels, as we didn't receive any bug reports.

As fixing this won't be hard, I ended opting to fix.

While here, I dropped an ugly hack that implemented read via
a separate function that was just doing a memcpy().

It should be noticed that, during the init phase, there's
a "b4" register that were never initialized, as its buffer
were used just to store the results of "b3" initialization.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>