i2c: brcmstb: Fix handling of optional interrupt

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>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>