logging: uart: cleanup device runtime PM checks

`pm_device_runtime_get` and `pm_device_runtime_put` have returned `0`
when device runtime PM is not enabled since #56222. Manually checking
the state is no longer required.

Additionally, the functions have been able to run in an ISR context
since #60785, which removed the need to special case `k_is_in_isr()`.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>