rtc: tegra: Propagate errors from platform_get_irq()

Instead of confusingly returning -EBUSY on failure to obtain an
interrupt, propagate the real error code. While at it, let the user know
why the interrupt could not be acquired.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>