m68k-semi.c: Use correct check for failure of do_brk()

In the m68k semihosting implementation of HOSTED_INIT_SIM, use the correct
check for whether do_brk() has failed -- it does not return -1 but the
previous value of the break limit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
