gve: Unobfuscate code by using nitems directly for loop.

While there, also make MODULE_PNP_INFO to reflect that the device
description is provided.

Reported-by: jrtc27
Reviewed-by: jrtc27, imp
Differential Revision: https://reviews.freebsd.org/D40430

(cherry picked from commit 1177a6c8dcfd6625c541d6ffc3c6915b8b6354f7)