target-mips: add missing check_dspr2 for multiply instructions

The emulator needs to check in hflags if DSP unit has been turned off before
it generates code for MUL_PH, MUL_S_PH, MULQ_S_W, and MULQ_RS_W.

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