[media] b2c2: Mismatch in config ifdefs for SkystarS2

Compilation warning issued by kbuild test robot:
>> drivers/media/common/b2c2/flexcop-fe-tuner.c:31:12: warning: 'flexcop_fe_request_firmware' defined but not used [-Wunused-function]
    static int flexcop_fe_request_firmware(struct dvb_frontend *fe,

This patch fixes a mismatch in Kconfig define checks. One had a
check for just CX24120, the other is checking for both CX24120
and ISL6421.

Signed-off-by: Jemma Denson <jdenson@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>