repart: cleanup use of ERRNO_IS_NOT_SUPPORTED()

repart: 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 arguments passed to ERRNO_IS_NOT_SUPPORTED() are the
values returned by efi_get_variable_string() and efi_set_variable()
which are not expected to return any positive values, but let's be
consistent anyway and move ERRNO_IS_NOT_SUPPORTED() invocations
to the branches where the return values are known to be negative.