spi: pxa2xx: missing break in pxa2xx_ssp_get_clk_div()

We refactored this code but accidentally left out a break statement so
QUARK_X1000_SSP isn't handled correctly.

Fixes: 025ffe88

 ('spi: pxa2xx: shift clk_div in one place')
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>