realtek: don't unmask non-maskable GPIO IRQs

realtek: don't unmask non-maskable GPIO IRQs

On uniprocessor builds, for_each_cpu(cpu, mask) will assume 'mask'
always contains exactly one CPU, and ignore the actual mask contents.
This causes the loop to run, even when it shouldn't on an empty mask,
and tries to access an uninitialised pointer.

Fix this by wrapping the loop in a cpumask_empty() check, to ensure it
will not run on uniprocessor builds if the CPU mask is empty.

Fixes: af6cd37f42f3 ("realtek: replace RTL93xx GPIO patches")
Reported-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Reported-by: Robert Marko <robimarko@gmail.com>
Tested-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Sander Vanheule <sander@svanheule.net>