Fix SPLIT_KEYBOARD compilation for ATMega*U2, which doesn't have VBUS/OTG control

[s-ol]

in split_util.c a code block #ifdef'd for AVR deals with specific USB features that only ATMega*U4 support, compilation on U2 fails like this:

quantum/split_common/split_util.c:77:5: error: implicit declaration of function 'USB_OTGPAD_On'; did you mean 'USB_REG_On'? [-Werror=implicit-function-declaration]
   77 |     USB_OTGPAD_On();  // enables VBUS pad
      |     ^~~~~~~~~~~~~
      |     USB_REG_On
quantum/split_common/split_util.c:80:12: error: implicit declaration of function 'USB_VBUS_GetStatus' [-Werror=implicit-function-declaration]
   80 |     return USB_VBUS_GetStatus();  // checks state of VBUS
      |            ^~~~~~~~~~~~~~~~~~

Description

This PR adjusts the ifdef to not attempt to use these facilities on ATmega16U2 and ATMega32U2 targets.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[fauxpark]

Since this is a bug, it can probably go into master.

[fauxpark]

Thanks!