mailbox: ZynqMP IPI: Delete an error message in zynqmp_ipi_probe()

The function platform_get_irq can log an error already.
Thus omit a redundant message for the exception handling in the
calling function.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>