fix(radio): remove invalid characters from logs filename.

[philmoz]

Fixes #5128

Logging uses model name to create log file - model name can include characters that are not valid in a filename on the radio.

[pfeerick]

Was just looking at this... perhaps worth doing as a helper function in sdcard.cpp? isFileAvailable() is there already... maybe either another function isValidFsChar()? that just does the letter by letter check or a sanitiseFilename() type one that does that entire check itself and returns a sanitised/valid filename? Are there will probably be other places we need to be doing this at some point... if not now.

[philmoz]

I've added a helper function in strhelpers.cpp so it can use the existing temp buffer.
Also cleaned up the log filename creation code.

[pfeerick]

LGTM on TPro with 5" Quad