ti/am335x: Fix the device_set_descf() call in am335x_dmtimer_probe()

Fixes: 459dc61c8b05 ("arm: Convert drivers to use device_set_desc(f)()")

Reviewed by: markj, imp, manu
Approved by: markj, imp, manu (mentor)
Differential Revision: https://reviews.freebsd.org/D45699