MdeModulePkg/PciBusDxe: Improve the flow of testing support attributes

MdeModulePkg/PciBusDxe: Improve the flow of testing support attributes

https://bugzilla.tianocore.org/show_bug.cgi?id=3635

Currently, in order to test the supported attributes,
the PciTestSupportedAttribute() will set the command register
to 0x27 (EFI_PCI_COMMAND_IO_SPACE, EFI_PCI_COMMAND_MEMORY_SPACE,
EFI_PCI_COMMAND_BUS_MASTER, EFI_PCI_COMMAND_VGA_PALETTE_SNOOP) firstly,
and then read back to check whether these attributes are
set successfully in the device.
This will cause the other enabled bits
 (other than EFI_PCI_COMMAND_IO_SPACE,EFI_PCI_COMMAND_MEMORY_SPACE,
 EFI_PCI_COMMAND_BUS_MASTER,EFI_PCI_COMMAND_VGA_PALETTE_SNOOP)
 be cleared for a short of time
 This patch fixes this issue by keeping the origina
 enabled bits when setting 0x27.

Signed-off-by: xueshengfeng <xueshengfeng@byosoft.com.cn>
Reviewed-by: Ray <ray.ni@intel.com>