MdeModulePkg/HiiDB: Handle the "&READONLY" tag in <KeywordResp> correctly

MdeModulePkg/HiiDB: Handle the "&READONLY" tag in <KeywordResp> correctly

This patch is to fix the incorrect logic when handling the "&READONLY" tag
in <KeywordResp>.
1. In UEFI spec, the "&READONLY" tag is in upper case, but using the lower
case in current codes by mistake.
2. The logic in checking the ReadOnly flag is not correct. Whether having
"&READONLY" tag must be consistent with the result of
"ExtractReadOnlyFromOpCode" function.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>