serial: sc16is7xx: Correct initialization of s->clk

The s->clk never gets setup in sc16is7xx_probe() and instead was using a
local clk variable, but then testing the uninitialized s->clk during
teardown

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jon Ringle <jringle@gridpoint.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>