target-mips: Fix helper and tests for dot/cross-dot product instructions

target-mips: Fix helper and tests for dot/cross-dot product instructions

Helper function for dpa_w_ph, dpax_w_ph, dps_w_ph and dpsx_w_ph incorrectly
defines halfword vector elements as unsigned values. This results in wrong
output which is not triggered in the tests as they also follow this logic.

Signed-off-by: Petar Jovanovic <petarj@mips.com>
Reviewed-by: Eric Johnson <ericj@mips.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>