drivers: flash: nrf5: Use faster write interval with BT sync

Use a lower slot duration than the used erase slot duration
to speed up the interval of write operations when in sync
with active Bluetooth connections.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>