Further cleanup to MIPS target read/write memory function:

- Move the mips32_..._read_mem calls to top-level of read_mem function.
- Change: Only perform mips_m4k_read_mem conversion when retval == ERROR_OK.  
   - Prevents pointless conversions of bogus read values after failures.
- Eliminate retval variable from mips_m4k_write_mem; return directly.
- Move declaration of retval variable to point of first use.
- Remove the now redundant switch statements testing size:
  - argument sanitizing already covers these cases.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2220 b42882b7-edfa-0310-969c-e2dbd0fdcd60