usb: ks_bridge: Check dev status in ksb_fs_release function

If the ksb device is disconnected before calling ksb_fs_release,
the device pointer may already be freed, which will lead to a
crash when calling dev_dbg. Fix this by checking the connection
status before calling dev_dbg.

CRs-fixed: 661871
Change-Id: I04c9e861f519f5fcbe57a8c4edf1038647c09783
Signed-off-by: Guang Jian Zeng <gzeng@codeaurora.org>