When an output port driver is removed, also remove references to it from
any masters. Failing to do this causes a NULL ptr dereference when
configuring another output port:

> BUG: unable to handle kernel NULL pointer dereference at 000000000000000d
> RIP: 0010:master_attr_store+0x9d/0x160 [intel_th_gth]
> Call Trace:
> dev_attr_store+0x1b/0x30
> sysfs_kf_write+0x3c/0x50
> kernfs_fop_write+0x125/0x1a0
> __vfs_write+0x3a/0x190
> ? __vfs_write+0x5/0x190
> ? _cond_resched+0x1a/0x50
> ? rcu_all_qs+0x5/0xb0
> ? __vfs_write+0x5/0x190
> vfs_write+0xb8/0x1b0
> ksys_write+0x55/0xc0
> __x64_sys_write+0x1a/0x20
> do_syscall_64+0x5a/0x140
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: b27a6a3f ("intel_th: Add Global Trace Hub driver")
CC: stable@vger.kernel.org # v4.4+
Reported-by: Ammy Yi <ammy.yi@intel.com>