Bugfixes in mips32_pracc.c

When testing a pic32mx220f032b with different values
for adapter_khz and cpu clocks i was getting a lot of
corrupted data from the chip. From time to time
openocd fails with segmentation faults or is aborted
due to memory corruption.

Change-Id: I134743f75c477b3d55dc74ae4474598e153b4a4a
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/690


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>