mailbox: arm_mhuv2: clean up loop in get_irq_chan_comb()

Both the inner and outer loops in this code use the "i" iterator.
The inner loop should really use a different iterator.

It doesn't affect things in practice because the data comes from the
device tree.  The "protocol" and "windows" variables are going to be
zero.  That means we're always going to hit the "return &chans[channel];"
statement and we're not going to want to iterate through the outer
loop again.

Still it's worth fixing this for future use cases.

Fixes: 5a6338cc ("mailbox: arm_mhuv2: Add driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>