e6000sw: Fix direct register write logic

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

Approved by: mw(mentor)
Obtained from: Semihalf
Sponsored by: Stormshield
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35672