USB: serial: cp210x: add new access functions for large registers

cp210x_get_config and cp210x_set_config are cumbersome to use. This change
switches large register access to use new block functions. The old
functions are removed because now they become unused.

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