fix(pm): add state check to soft off device suspension

[carrefinho]

Check device state before suspension during soft-off to avoid unexpected behavior from incorrectly suspended devices, using the same existing pattern in zmk_pm_suspend_devices.
Fixes high current consumption on XIAO nRF52840 upon wakeup from soft-off, likely caused by the QSPI flash being put in a weird state.

Tested on XIAO nRF52840 and XIAO nRF52840 Plus:

• Before: 4.6mA current draw after wake from soft-off (requires multiple additional resets to clear)
• After: Normal 40µA current draw after wake from soft-off.

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.

[sigvebh]

Works fine for me 👌