habanalabs: use registers name defines for ETR block

We have a single ETR block in the SOC, so use explicit register
name defines for initializing this block. This makes it more readable and
maintainable.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Omer Shpigelman <oshpigelman@habana.ai>