cmd: efidebug: fix a build error in show_efi_boot_opt()

I detected the following error in sandbox with Clang on Travis CI:
    +cmd/efidebug.c:703:15: error: result of comparison of constant
     9223372036854775822 with expression of type 'int' is always false
     [-Werror,-Wtautological-constant-out-of-range-compare]
    +        else if (ret == EFI_NOT_FOUND)
    +                 ~~~ ^  ~~~~~~~~~~~~~

Simply changing a type of 'ret' to efi_status_t will fix this error.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>