usb: serial: cp210x: Fix usage of uninitialized variable

In the function cp210x_get_termios_port, the variable
bits can be used unitialized if the cp210x_get_line_ctl
function call fails.
Fix this by initializing the variable to zero.

Change-Id: Ie520ecd818b782355412304a9bee403448b68c7a
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>