e6000sw: Fix register write logic

[kd]

Summary

When accessing a register directly from etherswitchcfg one must specify a register group(e.g. registers of portN) and the register offset within that group.
The latter is passed as the 5 least significant bits.
Extract the former by dividing the etherswitchcfg "register address" by 2^5=32, and not 5.

[mw]

Nice catch!