drivers: ieee802154: cc13xx_cc26xx_subg: simplify static funcs

Restructuring and renaming of driver-internal functions for improved
readability and maintainability:
- distinguish between externally exposed API methods
  (cc13xx_cc26xx_sub_* prefix) and internal helper methods
  (drv_* prefix).
- extract a few functions to reduce complexity and improve re-use

Also removes unnecessary initial runtime blanking of static (.bss) data
in the newly introduced extracted buffer initialization functions.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>