clocksource/drivers/ep93xx: Fix error handling during probe

When the interrupt property fails to be parsed, ep93xx_timer_of_init()
return code ends up uninitialized:

drivers/clocksource/timer-ep93xx.c:160:6: error: variable 'ret' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
        if (irq < 0) {
            ^~~~~~~
drivers/clocksource/timer-ep93xx.c:188:9: note: uninitialized use occurs here
        return ret;
               ^~~
drivers/clocksource/timer-ep93xx.c:160:2: note: remove the 'if' if its condition is always false
        if (irq < 0) {
        ^~~~~~~~~~~~~~

Simplify this portion to use the normal construct of just checking
whether a valid interrupt was returned. Note that irq_of_parse_and_map()
never returns a negative value and no other callers check for that either.

Fixes: c28ca80b ("clocksource: ep93xx: Add driver for Cirrus Logic EP93xx")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20231212214616.193098-1-arnd@kernel.org