mpr.4: Remove Broadcom LSI 3908 from support list

"This one is my fault due to incorrect merging" ~ziaee

MFC after: 3 days
Reviewed by: ziaee
Fixes: 744c8d554453 (mpr.4: Add Broadcom LSI 3808)
Differential Revision: https://reviews.freebsd.org/D53634