bootctl: cleanup use of ERRNO_IS_NOT_SUPPORTED()

bootctl: cleanup use of ERRNO_IS_NOT_SUPPORTED()

Given that ERRNO_IS_NOT_SUPPORTED() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the argument passed to ERRNO_IS_NOT_SUPPORTED() is the
value returned by efi_loader_get_entries() which is not expected to
return any positive values, but let's be consistent anyway and move
the ERRNO_IS_NOT_SUPPORTED() invocation to the branch where
the return value is known to be negative.