Handle BT Rx outside of pm_runtime with its own SICD index

There might be a corner case where host_isr wakes the system from
suspend and UART RTS goes low before nitrous_devcie_resume() gets
executed (since pm_runtime_get puts a work in queue to request the
device resume), but then goes low while a trnasfer is happening
because SICD_STATUS_BUSY was not set yet.

For that, handle SICD BUSY/IDLE through its own Rx index and move away
from pm_runtime routines, as this isn't needed anymore.

Test: Manual
Bug: 193466272

Signed-off-by: mullerf <mullerf@google.com>
Change-Id: I20ca5e865c4c100852166959abe055ddffc23075
Signed-off-by: jonerlin <jonerlin@google.com>