power: qpnp-bms: change irq number to signed integers

To allow the probe function to correctly check for errors change the
bms_irq struct's irq number from unsigned to signed integer.

This addresses some issues with static code checks.

Change-Id: Ie48a6475fd01a70ac0cf9a05b04c8fa79d29cc44
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>