EmbeddedPkg/PrePiLib: Prevent overread with FFS size macros

Symptom:Unsafe typecasting may lead to out‑of‑bound memory access

RootCause: FileSize and FileLength are declared as
UINT32 and masked with 0x00FFFFFF to store only the lower 24 bits.
Although this approach yields the correct result,
it introduces a potential risk due to unsafe typecasting and
dereferencing.

Solution: Using the predefined macro FFS_FILE_SIZE()
from MdePkg\Include\Pi\PiFirmwareFile.h,
which safely performs the same operation by reconstructing
the size using individual byte access.

This commit also addresses the fix for coverity isssue "OVERRUN"

Cc: Sachin Ganesh <sachinganesh@ami.com>
Signed-off-by: Gowtham M <gowthamm@ami.com>