target-s390x: fix CC computation for LOAD POSITIVE instructions

LOAD POSITIVE instructions (LPR, LPGR and LPGFR) set the following
condition code:
  0: Result zero; no overflow
  1: --
  2: Result greater than zero; no overflow
  3: Overflow

The current code wrongly returns 1 instead of 2 in case of a result
greater than 0. This patches fixes that. This fixes the marshalling of
the value '0L' in Python.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
