EmbeddedPkg/Lan9118Dxe: Do not return uninitialised TxBuff
Conform to the specification for GetStatus(), which states that "if
there are no transmit buffers to recycle and TxBuf is not NULL, *TxBuf
will be set to NULL".
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Michael Brown <mcb30@ipxe.org> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>