Read/write bit that allows software disable of laser.
Writing 1 disables laser. See Table 3.11 for enable/
disable timing requirements. This bit is OR d with the hard
TX_DISABLE pin value. Note, per SFP MSA TX_DISABLE pin is
default enabled unless pulled low by hardware. If Soft TX
Disable is not implemented, the transceiver ignores the value
of this bit. Default power up value is zero/low.
Byte 110, bit 6 of A2h