bus: fsl-mc: fsl-mc-allocator: Drop a write-only variable

Fixes a clang compiler warning:

   drivers/bus/fsl-mc/fsl-mc-allocator.c:565:6: warning: variable 'free_count' set but not used [-Wunused-but-set-variable]
           int free_count = 0;

Fixes: d8e026a4 ("staging: fsl-mc: remove some superfluous WARN_ONs")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Li Yang <leoyang.li@nxp.com>