USB: serial: rename cp2101 driver to cp210x

Lots of users are getting confused about the cp2101 driver.  It really
does support more than just the cp2101 device, so rename it to cp210x to
try to prevent confusion.

Cc: Craig Shelley <craig@microtron.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>