drivers: pwm: pwm_mcux_ctimer: use correct clock subsystem ID

With the merge of zephyrproject-rtos/zephyr/pull/70254,
MCUX_CTIMER_CLK_OFFSET is no longer defined nor needed. Remove this from
the macro getting the clock subsystem in the ctimer PWM driver.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>