The code to update the configuration register is repeated several times.
Move it into a separate function. At the same time, un-inline
lm90_select_remote_channel() and leave it up to the compiler to decide
what to do with it. Also remove the 'client' argument from
lm90_select_remote_channel() and from lm90_write_convrate() and take
it from struct lm90_data instead where needed.

This patch reduces code size by more than 800 bytes on x86_64.

Cc: Boyang Yu <byu@arista.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>