EDAC/xgene: Do not print a failure message to get an IRQ twice

Coccinelle reports a redundant error print in xgene_edac_probe() because
platform_get_irq() will already print an error message when it is unable
to get an IRQ.

Use platform_get_irq_optional() instead which avoids the error message
and keep the driver-specific one.

 [ bp: Sanitize commit message. ]

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Robert Richter <rric@kernel.org>
Link: https://lkml.kernel.org/r/20210112103540.7818-1-dong.menglong@zte.com.cn