Bluetooth: GATT: Fix invalid DB hast written to settings storage

Fix issue where an invalid (all zeroes) hash was written to settings
storage on reset. This caused the old value to written to zeroes, before
being written back to it's original value again immediately after.
This causes excessive flash wear.

This happens because the check if (k_delayed_work_remaining) returns the
amount of time until the work will execute. When that time has run out
the time is zero, but the work has not yet been executed.
We then write the invalid hash to flash, and then once the work-item
executes it will write the correct value.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>