i2c: brcmstb: The interrupt line is optional, so use platform_get_irq_optional

If there is no interrupt defined then an error is logged due
to the use of platform_get_irq. The driver handles not having
the interrupt by falling back to polling, therefore make
the appropriate call when claiming it.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>