[media] staging: as102: Remove leftovers of the SPI bus driver

SPI bus driver support is not included in this module, the SPI
driver files are missing. But some bits are still present so
clean up the unused code.
The SPI driver support can be properly added later if needed.

Then CONFIG_AS102_SPI and CONFIG_AS102_USB is now not needed
and the pre-processor statements using these config options
can now be removed from *.c files.

Cc: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Sylwester Nawrocki <snjw23@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>