[intelxl] Allow expected admin queue command errors to be silenced

[intelxl] Allow expected admin queue command errors to be silenced

The "clear PXE mode" admin queue command will return an EEXIST error
if the device is already in non-PXE mode, but there is no other admin
queue command that can be used to determine whether the device has
already been switched into non-PXE mode.

Provide a mechanism to allow expected errors from a command to be
silenced, to allow the "clear PXE mode" command to be cleanly used
without needing to first check the GLLAN_RCTL_0 register value.

Signed-off-by: Michael Brown <mcb30@ipxe.org>