net: wan: framer: pef2256: Switch to devm_mfd_add_devices()

net: wan: framer: pef2256: Switch to devm_mfd_add_devices()

The driver calls mfd_add_devices() but fails to call mfd_remove_devices()
in error paths after successful MFD device registration and in the remove
function. This leads to resource leaks where MFD child devices are not
properly unregistered.

Replace mfd_add_devices with devm_mfd_add_devices to automatically
manage the device resources.

Fixes: c96e976d9a05 ("net: wan: framer: Add support for the Lantiq PEF2256 framer")
Suggested-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Acked-by: Herve Codina <herve.codina@bootlin.com>
Link: https://patch.msgid.link/20251105034716.662-1-vulab@iscas.ac.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>