Fixes gcc '-Wunused-but-set-variable' warning:

drivers/pnp/isapnp/core.c: In function ‘isapnp_build_device_list’:
drivers/pnp/isapnp/core.c:777:27: warning: variable ‘checksum’ set
but not used [-Wunused-but-set-variable]

It is never used, and so can be removed.
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>