[ARM] footbridge: dc21285.c warning fixes

The dc21285 requests a number of IRQs that it doesn't really
care whether they get added. Change to use a macro that ensures
that at-least the user gets warned if they fail to add, which
also stops the warnings from __unused_result on request_irq().

dc21285.c:337: warning: ignoring return value of 'request_irq', declared with attribute warn_unused_result
dc21285.c:339: warning: ignoring return value of 'request_irq', declared with attribute warn_unused_result
dc21285.c:341: warning: ignoring return value of 'request_irq', declared with attribute warn_unused_result
dc21285.c:343: warning: ignoring return value of 'request_irq', declared with attribute warn_unused_result
dc21285.c:345: warning: ignoring return value of 'request_irq', declared with attribute warn_unused_result

Signed-off-by: Ben Dooks <ben-linux@fluff.org>