tty: serial: jsm: remove redundant pointer ch

Pointer ch is being assigned but is never used hence it is redundant
and can be removed.

Cleans up clang warning:
warning: variable 'ch' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>