mips32, use scan32 function for reading impcode/idcode.

There is no need to implement scan code in functions
mips_ejtag_get_idcode/impcode(), use mips_ejtag_drscan_32().
Impcode/idcode saved in ejtag.info.
Reorder the code in the callers of this functions.

Change-Id: Ia829c783a0b24c6a65cade736113fa6f67b0a170
Signed-off-by: Salvador Arroyo <salvador@telecable.es>
Reviewed-on: http://openocd.zylin.com/4003


Tested-by: jenkins
Reviewed-by: Peter Mamonov <pmamonov@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>