Fixes bug in tuh_msc_ready

[georgeboc]

Description
The documentation in msc_host.h specifies that:

// Check if the interface is currently ready or busy transferring data
bool tuh_msc_ready(uint8_t dev_addr);

An interface can be busy either receiving or sending data. The current code contemplates only the data reception, but not the data transmission.

See the following example for reference:

tuh_msc_read10(dev_addr, 0, &buffer, 0, 1, read_complete_cb, 0);

while (!tuh_msc_ready(dev_addr)) {
    tuh_task();
}

tuh_msc_inquiry(dev_addr, 0, &inquiry_resp, inquiry_complete_cb, 0);

while (!tuh_msc_ready(dev_addr)) {
    tuh_task();
}

The above example currently fails because the USB BULK transfers have no reserved bandwidth on the bus, so it is possible to have a race condition when the following steps happen:

1. send read command when calling tuh_msc_read10,
2. there is no bandwith available to send the command yet, but the processor continues,
3. we run the inquiry command when calling tuh_msc_inquiry,
4. we reach this line of code and we fail the assertion, as the interface is still busy trying to send the previous command.

This PR solves the problem by verifying in the tuh_msc_ready function that the output endpoint is not busy.

[hathach]

spot on, thank you for the fix