linuxkpi: Add I2C_NAME_SIZE and I2C_MODULE_PREFIX defines

Both are needed by drm-kmod

Obtained from: drm-kmod
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: //reviews.freebsd.org/D36212

(cherry picked from commit 7bf65b00c6d3ad2438e1bf40e4e76808a7bf5581)