MdePkg/PciExpress21.h: Fix typo in PCI_REG_PCIE_SLOT_CONTROL

MdePkg/PciExpress21.h: Fix typo in PCI_REG_PCIE_SLOT_CONTROL

PCI_REG_PCIE_SLOT_CONTROL contains a typo. It is defined as:
typedef union {
  struct {
    UINT32 AttentionButtonPressed : 1;
    UINT32 ...
    ...
  } Bits;
  UINT16   Uint16;
} PCI_REG_PCIE_SLOT_CONTROL;

The bit field data type should be UINT16 instead of UINT32,
results sizeof (PCI_REG_PCIE_SLOT_CONTROL) equals to 4 instead of 2.

Because this structure is used in PCI_CAPABILITY_PCIEXP as below:
typedef struct {
  ...
  PCI_REG_PCIE_SLOT_CONTROL       SlotControl;
  PCI_REG_PCIE_SLOT_STATUS        SlotStatus;
} PCI_CAPABILITY_PCIEXP;

It cause the OFFSET_OF (PCI_CAPABILITY_PCIEXP, SlotStatus) equal
to a wrong value.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>