pwm: axi-pwmgen: Create a dedicated function for getting driver data from a chip

Compared to direct calls to pwmchip_get_drvdata() a dedicated function
has two upsides: A better name and the right type. So the code becomes
easier to read and the new function is harder to use wrongly.

Another side effect (which is the secret motivation for this patch, but
shhh) is that the driver becomes a bit easier to backport to kernel
versions that don't have devm_pwmchip_alloc() yet.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: Trevor Gamblin <tgamblin@baylibre.com>
Link: https://lore.kernel.org/r/20240923125418.16558-2-u.kleine-koenig@baylibre.com


[ukleinek: added an * to the new function's prototype to make the compiler happy]
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>