[BUG] warnings building Bluefruit52Lib's blemidi example

[henrygab]

Describe the bug
Sample code currently emits warnings.

.../Bluefruit52Lib/examples/Peripheral/blemidi/blemidi.ino:
    In function 'void loop()':
.../Bluefruit52Lib/examples/Peripheral/blemidi/blemidi.ino:159:16:
    warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

   if (position >= sizeof(note_sequence)) {
       ~~~~~~~~~^~~~~~~~~~

Set up (please complete the following information)

• Using code from PR 376

To Reproduce
Steps to reproduce the behavior:

1. Start a build based on PR 376
2. Allow the build to complete
3. Expand one of the three build jobs that enable additional warnings
4. Review the build logs
   for the sample being fixed.

Expected behavior
Sample code should build without warnings.