ShellPkg: Fix calls to SimpleTextOut->SetAttribute to not use reserved bits

As the shell inverts foreground and background it needs to be sure that it properly masks off the reserved bits and this is important since the foreground and background are not the same numbers of bits.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <Jaben.carsey@intel.com>
Reviewed-by: Tapan Shah <tapandshah@hp.com>
Reviewed-by: Erik Bjorge <erik.c.bjorge@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/ShellPkg@15681 6f19259b-4bc3-4df7-8a09-765794883524