Minor bug fixes in Mips32 code

Minor bug fixes in Mips32 code

Now the the "Fast" version for memory blank check in pic32mx.c can be called:
default_flash_blank_check()  instead of the "fallback"  default_flash_mem_blank_check().

The command "verify_image", without working area, now don't show:
 checksum mismatch - attempting binary compare
when there are no real errors in flash.

Change-Id: I256e8ae949289634e1de5c1c2861e4c4c4b7fdce
Signed-off-by: Salvador <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/549
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>