hla: improve readability of struct hl_interface_s initialization

hla: improve readability of struct hl_interface_s initialization

The initialization is barely readable, while actually only few
fields are set with value nor zero nor NULL.

Rewrite the initialization using C99 struct designations.

Change-Id: I4d288e6536ebe7110a184db6540223fc67361ec3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6636
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>