efibootmgr: fix an incorrect error handling check

[mhorne]

Summary

efivar_device_path_to_unix_path() returns standard error codes on
failure and zero on success. Checking for a return value less than zero
means that the actual failure cases won't be handled. This could
manifest as a segfault during the subsequent call to printf().

Test Plan

With this patch applied, efibootmgr -E doesn't segfault on my system, and
instead gives:
efibootmgr: Can't convert to unix path