stand: eficom: Don't preemtively assume flow control

Remove rtsdtr_off. It's basically unused. Expand its meaning, but put
changing flow control to under an ifdef. We shouldn't set it unless
we're sure we need to do so. UEFI normally initializes the device
correctly, and we should avoid needless changes that aren't user
requested.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D40009