PM / devfreq: bw_hwmon: irq can be negative

platform_get_irq() can return a negative number, but we assign it
to an unsigned integer, which can never be negative. Change the
type to int here so that we can detect irq errors.

Change-Id: I997063abfe5c9966f99014a099619e6bfe7aafe7
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>