staging/ft1000: fix sparse warning about the kernel thread handler function

sparse throws warning about the ft1000_poll_thread as

drivers/staging/ft1000/ft1000-usb/ft1000_usb.c:40:5: warning: symbol 'ft1000_poll_thread' was not declared. Should it be static?

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>