Various fixes to the t4/5nex character device.

• Remove null open/close methods.
• Don't set d_flags to 0 explicitly.
• Remove t5_cdevsw as the .d_name member isn't really used and doesn't warrant a separate cdevsw just for the name.
• Use ENOTTY as the error value for an unknown ioctl request.
• Use make_dev_s() to close race with setting si_drv1.

Sponsored by: Chelsio Communications