zio-ad788x.c: re-organize code (and fix memory leakage)

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
zio-ad788x.c: split tx building in a new function

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
ad788x: add support to AD7476 1-chan ADC

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
USELESS NOW? buffers: do not pull_block if buffer is disabled

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
FIXME sysfs.c: flush: clear also user_block

Flushing after reading the last block corrupts nitem counter because the
user_block is still there until next read (write)

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>