tty: serial: jsm_tty: fixed redundant variable issue.

The variable "len" gets assigned once and it's value copied to
"n", which is then used for the rest of the function.

This patch fixes the unnecessary variable reassignment by using "len"
throughout the function instead.

Signed-off-by: Jakob Østergaard Jensen <jakob.jensen.91@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>