bootstd: Fix bootflow info for efi_mgr

bootstd: Fix bootflow info for efi_mgr

A "Synchronous Abort" CPU exception happens on an attempt to run the
"bootflow info" command for a global boot method (e.g. efi_mgr):

    => bootflow select 0
    => bootflow info

    "Synchronous Abort" handler, esr 0x96000006, far 0x8

It happens because do_bootflow_info() tries to dereference bflow->dev,
which is NULL in case of efi_mgr. Add the corresponding check to prevent
this NULL pointer dereference and make "bootflow info" command work
properly for global boot methods.

Fixes: 2d653f686b6d ("bootstd: Add a bootflow command")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>