sh: check return code of request_irq

request_irq is marked __must_check, but the call in shx3_prepare_cpus
has a void return type, so it can't propagate failure to the caller.
Follow cues from hexagon and just print an error.

Fixes: c7936b9a ("sh: smp: Hook in to the generic IPI handler for SH-X3 SMP.")
Cc: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Rich Felker <dalias@libc.org>