Fix issue #478.

[ghent360]

This PR fixes/implements the following bugs/features

• [ USBSerial loosing characters on Linux #478 ] Bug USBSerial loosing characters on Linux

In CDC_connected the core reads the value of the global variable transmitStart twice. This is a problem because the variable is modified inside ISR and the value can change between the two reads. It can lead to calculating the wrong value for transmitTime which can lead to loss of data.

This proposed fix would save the value of transmitStart in a local variable, so we read the value only once.

Fixes #478

[ktand]

@ghent360 @fpistm The fix is incorrect.

The line:

transmitTime = HAL_GetTick() - transmitStart;

should be

transmitTime = HAL_GetTick() - transmitTime;

otherwise you'll be reading transmitStart twice, as before.

[ghent360]

@ktand You are absolutely correct.

[ghent360]

I'm the dunce of the week. Can't make a 3 line code fix right.

[fpistm]

Thanks @ktand
Now, fixed in master.
I think I read too much code 🙄