[posix] clear SPI tx buffer after usage #55

Job	Run time
backbone-router	19m 28s
backbone-router	19m 28s
BR (mDNSResponder, TREL=0)	28m 57s
BR (mDNSResponder, TREL=0)	31m 22s
BR (mDNSResponder, TREL=1)	31m 9s
BR (mDNSResponder, TREL=1)	31m 9s
BR MATN (mDNSResponder, TREL=0)	21m 55s
BR MATN (mDNSResponder, TREL=0)	21m 55s
BR LowPower (mDNSResponder, TREL=0)	13m 33s
BR LowPower (mDNSResponder, TREL=0)	13m 33s
BR nat64 openthread (mDNSResponder, TREL=0)	26m 45s
BR nat64 openthread (mDNSResponder, TREL=0)	26m 45s
BR (avahi, TREL=0)	31m 4s
BR (avahi, TREL=0)	31m 4s
BR (avahi, TREL=1)	31m 7s
BR (avahi, TREL=1)	31m 7s
BR core firewall (avahi, TREL=0)	33m 28s
BR core firewall (avahi, TREL=0)	33m 28s
upload-coverage	0s
upload-coverage	1m 18s
delete-coverage-artifacts	0s
delete-coverage-artifacts	4s
	7h 58m 39s

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