MdeModulePkg/DxeNetLib: Fix negative value left shift

MdeModulePkg/DxeNetLib: Fix negative value left shift

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=698

Within function NetRandomInitSeed(), left shift a negative value is used
in:
"~Time.Hour << 24"

which involves undefined behavior.

Since Time.Hour is of type UINT8 (range from 0 to 23), hence ~Time.Hour
will be a negative value (of type int, signed).

According to the C11 spec, Section 6.5.7:
> 4 The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
>   bits are filled with zeros. If E1 has an unsigned type, the value
>   of the result is E1 * 2^E2 , reduced modulo one more than the
>   maximum value representable in the result type. If E1 has a signed
>   type and nonnegative value, and E1 * 2^E2 is representable in the
>   result type, then that is the resulting value; otherwise, the
>   behavior is undefined.

This commit will remove the '~' operator before 'Time.Hour', since it
seems like an implementation choice for generating the seed.

Cc: Steven Shi <steven.shi@intel.com>
Cc: Qin Long <qin.long@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>