collie_battery: Get rid of irq_to_gpio usage

Since 9d08d5d7, irq_to_gpio() is no
longer available but still in use by collie_battery.c. As it's just
for a debug message, just get rid of this call.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>