ShellPkg: Fix the incorrect behavior when pressing 'shift' key.
ShellPkg: Fix the incorrect behavior when pressing 'shift' key.

If 'ReadKeyStroke' function return EFI_NOT_READY then skip it.
If the return value is EFI_DEVICE_ERROR clean the currentString buffer.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Pedroa Liu <7ea4fd1ffb145e0b67db356517cf555d972e5299@insyde.com>
Reviewed-by: Qiu Shumin <a85ffd5e6451a83645d9ceae55f1cb6cdf90c130@intel.com>

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