pwm: img: fix pwm clock lookup

pwm: img: fix pwm clock lookup

22e8e19 has introduced a regression in the imgchip->pwm_clk lookup, whereas
the clock name has also been renamed to "imgchip". This causes the driver
failing to load:

[    0.546905] img-pwm 18101300.pwm: failed to get imgchip clock
[    0.553418] img-pwm: probe of 18101300.pwm failed with error -2

Fix this lookup by reverting the clock name back to "pwm".

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>
Link: https://lore.kernel.org/r/20240320083602.81592-1-wigyori@uid0.hu
Fixes: 22e8e19a46f7 ("pwm: img: Rename variable pointing to driver private data")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>