target-mips: fix mipsdsp_trunc16_sat16_round

This change corrects rounding and saturation of Q31 fractional value in
mipsdsp_trunc16_sat16_round(). Overflow detection was incorrect for the
corner case for PRECRQ_RS.PH, and this test case is also part of the change.

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
