IntelSiliconPkg IntelVTdDxe: Check HeaderType if func 0 is implemented

IntelSiliconPkg IntelVTdDxe: Check HeaderType if func 0 is implemented

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1169

Current code checks HeaderType of Function 0 even Function 0 is not
implemented. HeaderType value will be 0xFF if Function 0 is not
implemented, then MaxFunction will be set to PCI_MAX_FUNC + 1.

The code can be optimized to only check HeaderType if Function 0 is
implemented.

Test done:
With this patch, the result is same with the result after the patch at
https://lists.01.org/pipermail/edk2-devel/2018-September/029623.html.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Rangasai V Chaganty <rangasai.v.chaganty@intel.com>
Cc: Tomson Chang <tomson.chang@intel.com>
Cc: Jenny Huang <jenny.huang@intel.com>
Cc: Amy Chan <amy.chan@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@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>