USB: serial: ftdi_sio: only allow valid event_char values

The "event_char" device attribute value, when written, is interpreted as
an enable bit in bit 8, and an "event character" in bits 7 to 0.

Return an error -EINVAL for out-of-range values.  Use kstrtouint() 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>