wlcore: fix sparse warning

Use kstrtoul_from_user() for reading the user value,
and fix the following sparse warning:

drivers/net/wireless/ti/wlcore/debugfs.c:937:15: error: incompatible
types in comparison expression (different type sizes)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>