target-lm32: disable asm logging via LOG_DIS()

The lm32 target already has a disassembler which logs the assembly
instructions with "-d in_asm". Therefore, turn of the LOG_DIS() macro to
prevent logging the assembly instructions twice. Also turn the macro in a
one which is always compiled to catch any errors while the macro is turned
off.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>