fix(kscan): Remove warning when charlieplex is built without CONFIG_PM_DEVICE

[Nick-Munnich]

kscan_charlieplex_disconnect_all is only used within a CONFIG_PM_DEVICE block, so if the firmware is built without the flag it spits out a warning.

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.