OPENTHREAD-3844: Avoiding the direct access of the struct members

[silabs-HarshaK]

These changes are added to avoid the direct access of the members of struct in order to guarantee a consistent ABI across configurations.

[github-actions]

Merging #188 into main

name	branch	text	data	bss	total
ot-cli-ftd	e4479fb	480944	860	66580	548384
	920a299	480704	860	66580	548144
	+/-	-240	+0	+0	-240
ot-ncp-ftd	e4479fb	446868	764	61824	509456
	920a299	446628	764	61824	509216
	+/-	-240	+0	+0	-240
ot-cli-mtd	e4479fb	373120	764	51052	424936
	920a299	372880	764	51052	424696
	+/-	-240	+0	+0	-240
ot-ncp-mtd	e4479fb	354084	764	46320	401168
	920a299	353844	764	46320	400928
	+/-	-240	+0	+0	-240
ot-cli-ftd-br	e4479fb	584352	868	136412	721632
	920a299	584128	868	136412	721408
	+/-	-224	+0	+0	-224
ot-rcp	e4479fb	63136	568	20804	84508
	920a299	63136	568	20804	84508
	+/-	+0	+0	+0	+0

Library files

name	branch	text	data	bss	total
libopenthread-ftd.a	e4479fb	247855	95	40358	288308
	920a299	247889	95	40358	288342
	+/-	+34	+0	+0	+34
libopenthread-cli-ftd.a	e4479fb	61459	0	8083	69542
	920a299	61459	0	8083	69542
	+/-	+0	+0	+0	+0
libopenthread-ncp-ftd.a	e4479fb	33581	0	5948	39529
	920a299	33581	0	5948	39529
	+/-	+0	+0	+0	+0
libopenthread-mtd.a	e4479fb	165947	0	24854	190801
	920a299	165981	0	24854	190835
	+/-	+34	+0	+0	+34
libopenthread-cli-mtd.a	e4479fb	41414	0	8059	49473
	920a299	41414	0	8059	49473
	+/-	+0	+0	+0	+0
libopenthread-ncp-mtd.a	e4479fb	25805	0	5948	31753
	920a299	25805	0	5948	31753
	+/-	+0	+0	+0	+0
libopenthread-ftd-br.a	e4479fb	352577	100	110158	462835
	920a299	352611	100	110158	462869
	+/-	+34	+0	+0	+34
libopenthread-cli-ftd-br.a	e4479fb	79303	0	8115	87418
	920a299	79303	0	8115	87418
	+/-	+0	+0	+0	+0
libopenthread-rcp.a	e4479fb	9944	0	5060	15004
	920a299	9944	0	5060	15004
	+/-	+0	+0	+0	+0
libopenthread-radio.a	e4479fb	19411	0	238	19649
	920a299	19411	0	238	19649
	+/-	+0	+0	+0	+0