pwm: sti: Prefer local variable over pointer dereference

pwm: sti: Prefer local variable over pointer dereference

While the compiler probably optimizes out the pointer dereference, using
the local variable holding the same value also benefits the human
reader. So simplify accordingly. Also move a loop over all capture lines
into the capture if block to group the operations related to capturing
in .probe().

Link: https://lore.kernel.org/r/a7a81f3838f7ed7f4d6dbee3d646989cc265f676.1710068192.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>