staging: greybus: pwm: Rework how the number of PWM lines is determined

With a later patch it becomes necessary to already now the number of PWM
lines when pwmc is allocated. So make the function not use pwmc but a
plain connection and return the number of lines instead of storing it in
pwmc. This allows to get rid of the pwm_max member.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/3efd84ac03e7dc288f20b0de20b142b6404cb1fa.1707900770.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>