[secure-transport] simplify `Process()` method (#10933) #109

Job	Run time
thread-1-4-gcc-m32	11m 23s
thread-1-4-gcc-m64	11m 22s
channel-manager-csl	3m 30s
expects	31m 51s
thread-1-4-clang-m32	9m 42s
thread-1-4-clang-m64	9m 41s
packet-verification-1-1-on-1-4	12m 28s
packet-verification-low-power	10m 28s
thread-1-4-posix	9m 19s
upload-coverage	23s
	1h 50m 7s

Provide feedback