drivers: flash: nRF5x: Fix co-operation with Bluetooth controller

Fixed incorrect comparison of FLASH_SLOT in microseconds
with ticks elapsed.

This caused Bluetooth controller to try scheduling events
beyond the acceptable soft real-time design limits, due to
the CPU halted for flash erase operation.

Fixes #6229.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>