vf_i2c: Make ext_resources non-optional

EXT_RESOURCES have been introduced in 12-CURRENT and all supported
releases have it enabled in their kernel config.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33832

(cherry picked from commit 4bcdfeb06fc7e7463904a8a89d7e45b277f21483)