efibootmgr: fix an incorrect error handling check

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().

Reviewed by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D27424