Update STM DFU mode software implementation.

[flom84]

Hello,

I found a couple of issues with my first implementation of the STM DFU mode #6919

1. the size of the interrupt clear array was not computed correctly
2. the recommendation to disable interrupts using NVIC from ARM (nested vector interrupt controller) was not followed

I retested with my STM32 Nucleo-F446RE board, attached picture bellow:

[jepler]

Thanks! One note, if you want to change it before I merge this PR, you could consider use of MP_ARRAY_SIZE. If not, can merge as-is too.

[jepler]

I looked at this many times before I spotted the problem.

In CircuitPython, you can use MP_ARRAY_SIZE(arr) (in py/misc.h) to compute the size of the array. Note that if the expression arr is actually a pointer instead, it will NOT be detected as an error. (the implementation is just sizeof(arr) / sizeof(arr[0]), more or less)

[flom84]

@jepler did the changes. Testing now.

[jepler]

Thanks again!

[flom84]

@jepler thank you !