ShellPkg: Fix bad TimeZone (TZ) conversion.

ShellPkg: Fix bad TimeZone (TZ) conversion.

EFI_UNSPECIFIED_TIMEZONE means display local time. TZ of 0 is UTC.
Thus EFI_UNSPECIFIED_TIMEZONE means ignore TZ, 0 means UTC. When this code is
fixed to adust file TZ to local TZ you need to preserve
EFI_UNSPECIFIED_TIMEZONE.

FAT always return EFI_UNSPECIFIED_TIMEZONE.
Modern filesystems, HFS+, NTFS, ext3, etc store time in UTC.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Andrew Fish <afish@apple.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18051 6f19259b-4bc3-4df7-8a09-765794883524