i2c: ocores: Avoid false-positive error log message.

Since commit 7723f4c5

 ("driver core: platform: Add an error message to
platform_get_irq*()"), platform_get_irq() will call dev_err() on an error.
In case of i2c ocores this leads to a false-positive error being logged:

[ 4.173989] 007: ocores-i2c ocores-i2c: IRQ index 0 not found

i2c ocores already handles the case when the IRQ cannot be found and
therefore there is no benefit in having this error message being logged.

This commit switches to platform_get_irq_optional(), which does not log
in case the IRQ cannot be found.

Signed-off-by: Stefan Lässer <stefan.laesser@omicronenergy.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>