ShellPkg/UefiShellDebug1CommandsLib: remove I/O address limit from 'mm'

ShellPkg/UefiShellDebug1CommandsLib: remove I/O address limit from 'mm'

Neither the EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL nor the EFI_CPU_IO2_PROTOCOL
impose any restrictions when it comes to the range of valid I/O
addresses. Even so, the 'mm' command in -IO mode refuses to perform
accesses to addresses >= 0xffff.

It is not up to 'mm' to impose this restriction, so remove it.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>