MdeModulePkg/UdfDxe: Correct behavior for UdfSetPosition()

MdeModulePkg/UdfDxe: Correct behavior for UdfSetPosition()

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1252

Calling the 'SetPosition' service of the EFI_FILE_PROTOCOL with 'Position'
equals to 0xFFFFFFFFFFFFFFFF for a file is to set the current position to
the end of the file. But the current implementation of function
UdfSetPosition() is to set it to the last byte (not EOF).

This commit will resolve this issue.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Paulo Alcantara <palcantara@suse.de>
Acked-by: Star Zeng <star.zeng@intel.com>