[spinel] replace otLinkGetFrameCounter with otPlatRadioGetFrameCounte… #19

Job	Run time
backbone-router	3m 42s
BR (mDNSResponder, TREL=0)	3m 48s
BR (mDNSResponder, TREL=1)	4m 2s
BR MATN (mDNSResponder, TREL=0)	4m 4s
BR LowPower (mDNSResponder, TREL=0)	3m 39s
BR internet access (mDNSResponder, TREL=0)	3m 42s
BR (avahi, TREL=0)	3m 58s
BR (avahi, TREL=1)	3m 45s
BR core-firewall (avahi, TREL=0)	4m 2s
upload-coverage	0s
	34m 42s

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