Input: rmi4 - change reg_size type to u32

Input: rmi4 - change reg_size type to u32

Change reg_size from unsigned long to u32 to save space and ensure
consistent size across 32-bit and 64-bit architectures, and use
DECLARE_BITMAP() for subpacket_map.

Also pack the structure by rearranging the members to avoid holes,
and use size_add() to prevent potential integer overflows when
calculating the total size of registers.

Assisted-by: Gemini:gemini-3.1-pro
Link: https://patch.msgid.link/20260505045952.1570713-13-dmitry.torokhov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>