ARM: at91: unused variable in at91_pm_verify_clocks

The code using the variable 'i' in this function is conditional which
results in a harmless compiler warning. Using the IS_ENABLED macro
instead of #ifdef makes the code look nicer and gets rid of the
warning.

Without this patch, building at91sam9263_defconfig results in:

/home/arnd/linux-arm/arch/arm/mach-at91/pm.c: In function 'at91_pm_verify_clocks':
/home/arnd/linux-arm/arch/arm/mach-at91/pm.c:137:6: warning: unused variable 'i' [-Wunused-variable]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>