drivers/power/pm8921-charger.c: Fix ISO C90 compile error

drivers/power/pm8921-charger.c: In function 'update_heartbeat':
drivers/power/pm8921-charger.c:3715:2: warning: ISO C90 forbids
mixed declarations and code [-Wdeclaration-after-statement]
error, forbidden warning: pm8921-charger.c:3715

Change-Id: Id31b560f4b86155ec380562b39fac4fc87dbafb7