Hexagon (target/hexagon) fix bug in conv_df2uw_chop

Fix typo that checked for 32 bit nan instead of 64 bit

Test case added in tests/tcg/hexagon/usr.c

Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Message-Id: <20220210021556.9217-11-tsimpson@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>