usb: musb: dsps: move debugfs_remove_recursive()

When the platform initialization fails due to missing resources, it will
return -EPROBE_DEFER after dsps_musb_init() has been called.

dsps_musb_init() calls dsps_musb_dbg_init() to allocate the debugfs
nodes. At a later point in time, the probe will be retried, and
dsps_musb_dbg_init() will be called again. debugfs_create_dir() will
fail this time, as the node already exists, and so the entire device
probe will fail with -ENOMEM.

Fix this by moving debugfs_remove_recursive() from dsps_remove() to the
plaform's exit function, so it will be cleanly torn down when the probe
fails. It also feels more natural this way, as .exit is the counterpart
to .init.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>