When the stm32 pwm driver is converted to pwmchip_alloc(), the number of
available PWM lines must be known before the driver private data can be
allocated. So rework the helper function that determines this number to
not take the driver private data struct as input parameter.

Link: https://lore.kernel.org/r/13d4d3e90a9ee1bcd04674dfdc16f242615b8320.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>