e1000: disable hw.em.sbp debug setting

This is a debugging tunable that shouldn't have retained this setting
after the initial iflib conversion of the driver

PR: 248934
Reported by: Franco Fichtner <franco@opnsense.org>
Reviewed by: markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D29768

(cherry picked from commit 548d8a131d536d5f8e0818ff8cff7ffd63dbedfe)