proc: Use underscores for SSBD in 'status'

The style for the 'status' file is CamelCase or this. _.

Fixes: fae1fa0f ("proc: Provide details on speculation flaw mitigations")
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>