netfront: fix the support for disabling LRO at boot time

The driver has a tunable hw.xn.enable_lro which is intended to control
whether LRO is enabled. This is currently non-functional - even if its
set to zero, the driver still requests LRO support from the backend.
This change fixes the feature so that if enable_lro is set to zero, LRO
no longer appears in the interface capabilities and LRO is not requested
from the backend.

PR: 273046
MFC after: 1 week
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D41439

(cherry picked from commit da4b0d6eb06d730487d48e15d2d5e10c56266fd9)