EmbeddedPkg/GdbSerialLib: avoid left shift of negative quantity

EmbeddedPkg/GdbSerialLib: avoid left shift of negative quantity

Clang complains about left shifting a negative value being undefined.

  EmbeddedPkg/Library/GdbSerialLib/GdbSerialLib.c:151:30:
  error: shifting a negative signed value is undefined [-Werror,-Wshift-negative-value]
  OutputData = (UINT8)((~DLAB<<7)|((BreakSet<<6)|((Parity<<3)|((StopBits<<2)| Data))));

Redefine all bit pattern constants as unsigned to work around this.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>