spi/bcm63xx-hsspi: keep pll clk enabled

If the pll clock needs to be enabled to get its rate, it will also need
to be enabled to provide it. So ensure it is kept enabled through the
lifetime of the device.

Fixes: 0d7412ed ("spi/bcm63xx-hspi: Enable the clock before calling clk_get_rate().")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>