spi: bcm63xx-hsspi: fix pm_runtime

The driver sets auto_runtime_pm to true, but it doesn't call
pm_runtime_enable(), which results in "Failed to power device" when PM support
is enabled.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Link: https://lore.kernel.org/r/20210223151851.4110-3-noltari@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>