ChibiOS: fix deadlocks when hammering mouse/extra keys

[lactide]

When hammering on the mouse or extra keys, tmk will deadlock.
To fix that, i've copied the fix from the keyboard endpoint [1]

[1] tmk/tmk_keyboard@72c52d3

[lactide]

I'm not sure anymore whether this was the correct fix in the first place.

tmk only "deadlocks" because i've enabled ChibiOS assertions. This particular assertion in usbTransmitI fires if the last packet hasn't been acknowledged yet by the host and tmk wants to send the next report.
So this commit only helps if the host still acknowledges the reports.

But this isn't always the case. My FreeBSD installation doesn't seem to understand the extra_keys or the nkro reports and won't acknowledge any of them.
Same issue in the BIOS of a thinkpad x201 for mouse or extra keys. So if the host doesn't acknowledge any packets, then the patch above will still trigger the assertion.

ChibiOS calls a callback when the last transmission is finished.
So if we want to transmit a report and the endpoint still hasn't sent the last report, we'll keep a pointer to the new report and will send it if the callback of the endpoint is called. I've implemented this in my branch and force pushed the commit.