spi: bcm2835: change initialization order and switch to platform_get_irq

Change the initialization order of the HW so that the interrupt
is only requested after the HW is initialized

Also the use of irq_of_parse_and_map is replaced by platform_get_irq.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>