MdeModulePkg: Rename confusion function name

MdeModulePkg: Rename confusion function name

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1536

To avoid the confusion caused by function name,
rename EfiBootManagerGetNextFullDevicePath to
EfiBootManagerGetNextLoadOptionDevicePath.
As an API function should add EFIAPI prefix for this function.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>