i2c: sh7760: add IRQ check

The driver neglects to check the result of platform_get_irq()'s call and
blithely passes the negative error codes to devm_request_irq() (which
takes *unsigned* IRQ #), causing it to fail with -EINVAL, overriding
an original error code.  Stop calling devm_request_irq() with invalid
IRQ #s.

Fixes: a26c20b1

 ("i2c: Renesas SH7760 I2C master driver")
Signed-off-by: Sergey Shtylyov <s.shtylyov@omprussia.ru>
Signed-off-by: Wolfram Sang <wsa@kernel.org>