spi: bcm2835: fill FIFO before enabling interrupts to reduce interrupts/message

To reduce the number of interrupts/message we fill the FIFO before
enabling interrupts - for short messages this reduces the interrupt count
from 2 to 1 interrupt.

There have been rare cases where short (<200ns) chip-select switches with
native CS have been observed during such operation, this is why this
optimization is only enabled for GPIO-CS.

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