IntelSiliconPkg IntelVTdDxe: Fix incorrect code to clear VTd error

IntelSiliconPkg IntelVTdDxe: Fix incorrect code to clear VTd error

According to VTd spec, Software writes the value read from this
field (F) to Clear it. But current code is using 0 to clear the
field, that is incorrect.

And R_FSTS_REG register value clearing should be not in the for loop.

Without this patch, we will see same VTd error message appears again
and again after it occurs first time.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>