ASoC: bcm2835: setup clock only if CPU is clock master

We only need to enable the clock if we are a clock master.

Code ported from bcm2708-i2s driver in Raspberry Pi tree.
Original work by Zoltan Szenczi.

Signed-off-by: Matthias Reichl <hias@horus.com>
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>