x86: hpet: Fix masking of MSI interrupts

HPET_TN_FSB is not a proper mask bit; it merely toggles between MSI and
legacy interrupt delivery. The proper mask bit is HPET_TN_ENABLE, so
use both bits when (un)masking the interrupt.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/5093E09002000078000A60E6@nat28.tlf.novell.com


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