MdeModulePkg:Change the type of BootNext

MdeModulePkg:Change the type of BootNext

Currently the invalid boot next set to the number of boot option,
when add a new boot option,also need update the boot next value,
otherwise it will be incorrect.Now set the type of BootNext value
to UINT32,the number out of the range of UINT16 means it is an
invalid BootNext Value.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19591 6f19259b-4bc3-4df7-8a09-765794883524