dwmmc: 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/D33822

(cherry picked from commit b4cc0f7d28e2f4ae12db3be75658ac5b8c1293f1)