wcnss: Export SMD channel close API for wlan driver removal

The SMD channel is not the primary WCNSS channel and must explicitly be
closed as the device is removed, or the channel will already be open on
a subsequent probe call in e.g. the case of reloading the
wlan kernel module.

Export SMD channel close API for wlan driver removal case.

Change-Id: I2f140c429d7c57b8dd53e4e62c9e4dc12b42bf8e
Signed-off-by: Govind Singh <govinds@codeaurora.org>