rpmsg: smd: fix dependency on QCOM_SMD=n

The ARM allmodconfig build broke with the addition of the SMD rpmsg
driver that conflicts with the driver its replaces:

WARNING: drivers/soc/qcom/smd: 'qcom_smd_register_edge' exported twice. Previous export was in drivers/rpmsg/qcom_smd.ko
WARNING: drivers/soc/qcom/smd: 'qcom_smd_unregister_edge' exported twice. Previous export was in drivers/rpmsg/qcom_smd.ko

There is already a dependency that is meant to avoid the broken
configuration, but that only prevents the case where at least
one of the two are built-in, but not if both are modules.

This changes the dependency to "=n", to ensure that the new driver
can only be enabled if the other one is completely disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 53e2822e ("rpmsg: Introduce Qualcomm SMD backend")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>