x86: distangle user disabled TSC from unstable

tsc_enabled is set to 0 from the command line switch "notsc" and from
the mark_tsc_unstable code. Seperate those functionalities and replace
tsc_enable with tsc_disable. This makes also the native_sched_clock()
decision when to use TSC understandable.

Preparatory patch to solve the sched_clock() issue on 32 bit.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>