pwm: atmel-tcb: Simplify checking the companion output

pwm: atmel-tcb: Simplify checking the companion output

The two outputs provided by the supported hardware share some settings,
so access to the other PWM is required when one of them is configured.

Instead of an explicit if to deterimine the other PWM just use
hwpwm ^ 1. Further atcbpwm is never NULL, so drop the corresponding
check.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20240709101806.52394-4-u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>