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.

Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>
(cherry picked from commit acd4c77f)

Change-Id: I6b78092d0dc687184c9a12203441085382fd5c1a
Signed-off-by: Sudhir Sharma <sudsha@codeaurora.org>