MdeModulePkg/HiiDatabaseDxe: ASSERT "Private->Attribute >> 4"

MdeModulePkg/HiiDatabaseDxe: ASSERT "Private->Attribute >> 4"

Foreground and background color are saved in a single byte.
Bits 0..3 are the foreground color and bits 4..6 are the background color.
If the Private->Attribute defined correctly, (Private->Attribute >> 4)
must be less than 8.
This commit uses ASSERT to assume "Attribute >> 4" is less than 8.

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Dandan Bi <dandan.bi@intel.com>