USB: serial: ti_usb_3410_5052: do not use __uX types

__uX types should only be used for user-space interactions.

Also clean up uart-config endianess handling, and drop some redundant
casts.

Signed-off-by: Mathieu OTHACEHE <m.othacehe@gmail.com>
[ johan: amend commit message ]
Signed-off-by: Johan Hovold <johan@kernel.org>