pwm: atmel-tcb: fix max time computation for slow clk source

Use the the tcb counter width to compute the maximum time that can be
represented using the slow clock source instead of the static 16 bit width.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>