Local backend: Return -errno instead of -EBUSY when a write error occurs

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Local backend: Delay buffer enabling when using the low-speed interface

When using the low-speed interface, the buffer is now enabled before the
first buffer is read (in case of an input buffer) or after the first
buffer is written (in case of an output buffer).

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>