usb: gadget: FunctionFS: Use kstrtoul()

kstrtoul() checks for overflow which simple_strtoul() does not pluss
it has “*end == 0” check in it as well.  As a side effect, a new
line character is now accepted, but this should not be an issue.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>