openrisc: fix clang error core.CallAndMessage

openrisc: fix clang error core.CallAndMessage

Clang assumes that size could assume a value that is not 1 nor 2
nor 4. In such condition the buffer in t is allocated (size != 1)
and not initialized. This triggers an error:
  src/target/openrisc/or1k_du_adv.c:655:14: warning: 2nd function
    call argument is an uninitialized value [core.CallAndMessage]
                crc_calc = adbg_compute_crc(crc_calc, data[i], 8);
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Add the default case to cover other values of size.

After this fix, clang still complains on the same line, this time
misunderstanding the limits of the loop and considering that
buf_bswap16() only swaps the first 16 bits, thus passing not
initialized value data[2] to adbg_compute_crc()

Replace malloc() with calloc() to silent it.

Change-Id: I358d7fb2ebefd69255670641bd435b770762a301
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7300
Tested-by: jenkins