EmbeddedPkg/TimeBaseLib: remove useless truncation to 32-bit

EmbeddedPkg/TimeBaseLib: remove useless truncation to 32-bit

EfiTimeToEpoch() calls EfiGetEpochDays() internally, which (reasonably)
returns a UINTN. But then EfiTimeToEpoch() truncates the EfiGetEpochDays()
retval to UINT32 for no good reason, effectively restricting Time->Year
under 2106.

This truncation was pointed out with a valid warning (= build error) by
VS2019.

Allow EfiTimeToEpoch() to return / propagate a UINTN value.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201221113657.6779-3-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>