MBO: Fix minimum length check on non_pref_chan configuration

MBO: Fix minimum length check on non_pref_chan configuration

The reason detail field in non_pref_chan attribute was removed
from MBO draft v0.0_r25. Also oper_class can be 1 character for
few country codes (e.g., country code-UK, channel number-1). So the
shortest channel configuration is 7 characters.

This was missed in the earlier commit
4a83d4b6861f6627b6b256b8c126547a19409a70 ('MBO: Do not add reason_detail
in non_pref_chan attr (STA)') that took care of other changes related to
removal of the reason detail.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>