net: gptp: kconfig: Fix log-based intervals help

Fix the formula listed in help messages which is used to calculate
actual intervals given their log2 values.

It is all calculated properly in the code, because the unit of the
actual field uses the UScaledNs type of which the unit is 2^(-16)ns, so
it is just the help messages that got it wrong.

Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>