tty: n_smux: Increased smux log buffer size

Increase log buffer size to handle the maximum possible log line size.
This prevents passing in a negative buffer size to sprintf() which is
handled gracefully, but generates a warning message in the kernel log.

Change-Id: Ie87fe3a5431757217f4b87338d5af08bc24d84e6
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>