Input: rmi4 - refactor register descriptor parsing

Input: rmi4 - refactor register descriptor parsing

Factor out parsing a register descriptor item from
rmi_read_register_desc() and ensure there are no out-of-bounds accesses.

Use get_unaligned_le16() and get_unaligned_le32() for reading multi-byte
values.

Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: 2b6a321da9a2 ("Input: synaptics-rmi4 - add support for Synaptics RMI4 devices")
Cc: stable@vger.kernel.org
Assisted-by: Gemini:gemini-3.1-pro
Link: https://patch.msgid.link/20260505045952.1570713-2-dmitry.torokhov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>