Fix issues found using default CodeQL settings.

[henrygab]

Describe the PR

This fix does the following:

1. Fix the CodeQL alert by changing the loop variable type to use int.
2. Explicitly limiting the loop to MAXIMUM_TOTAL_DRIVER_COUNT.
3. Sets the originally named variable to the loop variable (but c-style cast to type uint8_t).

In addition, because there was not previously any code to ensure that TOTAL_DRIVER_COUNT would be in the expected range of [0x01..0xFF], added a TU_ASSERT() at the appropriate location to ensure this condition, and immediately stop execution if a configuration would ever exceed this.

There is no change to the logical flow, except for that additional TU_ASSERT().

What is solved

When relying projects enable CodeQL (with default settings), and build tinyUSB,
ten (10x) high-severity alerts of the following type will be generated:

Comparison of narrow type with wide type in loop condition

This PR updates the code to prevent these CodeQL alerts,
which otherwise show up in all projects relying on TinyUSB.

Additional context

The alerts note that the loop variable (e.g., i) is of a smaller size (e.g., uint8_t) than what it is being compared against (e.g., int). As an example, code which would generate this alert is:

for (uint8_t i = 0; i < TOTAL_DRIVER_COUNT; i++) {
    usbd_class_driver_t const* driver = get_driver(i);
    // ...
}

The type of TOTAL_DRIVER_COUNT is int (uint8_t + size_t --> promoted to int).
The type of the loop variable was uint8_t.
Therefore, this would loop infinitely if TOTAL_DRIVER_COUNT ever exceeds 0xFFu.
As a result, a high severity alert is generated by CodeQL.

It is understood that the count of drivers is low (built-in is only ~12 right now), and unlikely to ever exceed 0xFFu.
However, there was no code that ensured that TOTAL_DRIVER_COUNT would be in the expected range of [ 0x01u .. 0xFFu ].