target/mips: Drop redundant gen_io_start/stop()

DMTC0 CP0_Cause does a redundant gen_io_start() and gen_io_end() pair,
even though this is done for all DMTC0 operations outside of the switch
statement. Remove these redundant calls.

Fixes: 5dc5d9f0 ("mips: more fixes to the MIPS interrupt glue logic")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Yongbok Kim <yongbok.kim@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>