eficonfig: fix missing variable initialization

The 'ret' variable must be initialized before use
in eficonfig_delete_invalid_boot_option().

Fixes: c416f1c0 ("bootmenu: add removable media entries")
Addresses-Coverity: 376207 ("Uninitialized variables")
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>