Staging: tm6000: remove unneeded check in get_next_buf()

We dereference "buf" on the line before so if it were NULL here we
would have OOPsed earlier.  Also list_entry() never returns NULL.
And finally, we handled the situation where the list is empty
earlier in the function.

So this test isn't needed and I've removed it.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>