can: m_can: hrtimer_callback(): rename to m_can_polling_timer()

can: m_can: hrtimer_callback(): rename to m_can_polling_timer()

The original use of struct m_can_classdev::hrtimer was to support polling
for devices without IRQ, with the timer function called hrtimer_callback().

Commit 07f25091ca02 ("can: m_can: Implement receive coalescing") uses the
hrtimer for software-supported IRQ coalescence, with the timer function
called m_can_coalescing_timer().

To improve the readability of the driver, rename hrtimer_callback() to
m_can_polling_timer(), which better describes the functionality.

Link: https://patch.msgid.link/20251008-m_can-cleanups-v1-2-1784a18eaa84@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>