Staging: sst: user pointers in intel_sst_mmap_play_capture()

There were some places in intel_sst_mmap_play_capture() that
dereferenced user pointers instead of copying the data to the kernel.

I removed the BUG_ON(!mmap_buf) and BUG_ON(!buf_entry) since those are
never possible in the current code.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>