spi/bcm63xx-hspi: Enable the clock before calling clk_get_rate().

Enable the clock prior to calling clk_get_rate(), because clk_get_rate()
should only be called if the clock is enabled.

Additionally, prepare/enable the pll_clk before calling clk_get_rate()
for the same reason.

Found by Linux Driver Verification project (linuxtesting.org).

Fixes: 142168eb ("spi: bcm63xx-hsspi: add bcm63xx HSSPI driver")
Signed-off-by: Stefan Potyra <Stefan.Potyra@elektrobit.com>
Signed-off-by: Mark Brown <broonie@kernel.org>