input: edt-ft5x06: Correct prefix length in snprintf

snprintf takes the length of the array that we can print into,
and has to fit the NULL terminator in there too.
Printing the prefix is generally "12-3456 " which is 8 desired
characters (the length of EDT_NAME_PREFIX_LEN) and the NULL.
The space is therefore being truncated to fit the NULL in.

Increase the length snprintf is allowed to use.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>