net: wifi: Use micro seconds for precision for TWT intervals

In order to take granular input use micro seconds as input for TWT
intervals, this helps us in providing inputs such as 65.28ms without the
need of using floating points.

This also expands the TWT wake interval range to 262.144ms, earlier as
we want to use uint8, limited to 256ms.

Also, remove the units from the variable names, this is unnecessary and
also avoids doing breaking changes.

Update release notes as this is a breaking change, both type and
variable names are changed.

Signed-off-by: Krishna T <krishna.t@nordicsemi.no>