USB: serial: cp210x: add 8-bit and 32-bit register access functions

cp210x_get_config and cp210x_set_config are cumbersome to use. This change
introduces new register access functions for 8 and 32-bit values, instead
of the above functions.

Signed-off-by: Konstantin Shkolnyy <konstantin.shkolnyy@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>