fix(keymap): Fix overflow in get/set bindings

[ReFil]

With the binding referenced to as uint8_t the keymap would be incorrect if more than 256 positions were in use as they would overflow back to position 0, going to a uint16_t gives a significantly improved headroom

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.

[petejohanson]

Thanks!