i2c: xlp9xx: fix main IRQ check

Iff platform_get_irq() returns 0 for the main IRQ, the driver's probe()
method will return 0 early (as if the method's call was successful).
Let's consider IRQ0 valid for simplicity -- devm_request_irq() can always
override that decision...

Fixes: 2bbd681b ("i2c: xlp9xx: Driver for Netlogic XLP9XX/5XX I2C controller")
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: George Cherian <george.cherian@marvell.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>