i2c: highlander: add IRQ check

The driver is written as if platform_get_irq() returns 0 on errors (while
actually it returns a negative error code), blithely passing these error
codes to request_irq() (which takes *unsigned* IRQ #) -- which fails with
-EINVAL. Add the necessary error check to the pre-existing *if* statement
forcing the driver into the polling mode...

Fixes: 4ad48e6a ("i2c: Renesas Highlander FPGA SMBus support")
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Wolfram Sang <wsa@kernel.org>