USB: serial: oti6858: remove redundant assignment to variable divisor

The variable divisor is being assigned a value that is not being
read afterward, it is being re-assigned later in both paths of an if
statement.  The assignment is redundant and can be removed.

Cleans up clang scan warning:
drivers/usb/serial/oti6858.c:412:2: warning: Value stored to 'divisor'
is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>