USB: serial: ftdi_sio: only allow valid latency timer values

USB: serial: ftdi_sio: only allow valid latency timer values

Valid latency timer values are between 1 ms and 255 ms in 1 ms steps.
The store function for the "latency_timer" device attribute currently
allows any value, although only the lower 16 bits will be sent to the
device, and the device only stores the lower 8 bits.  The hardware
appears to accept the (invalid) value 0 and treats it the same as 1
(resulting in a latency of 1 ms).

Change the latency_timer_store() function to accept only the values 0 to
255, returning an error -EINVAL for out-of-range values.  Call
kstrtou8() to parse the integer instead of the obsolete
simple_strtoul().

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Johan Hovold <johan@kernel.org>