i2c: expand minor range when registering chrdev region

Actually, total amount of available minor number for a single major is
MINORMASK + 1. So expand minor range when registering chrdev region.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
[wsa: fixed typo in commit message]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>