rsi: use macro for allocating USB buffer

4 bytes is fixed size for reading or writing USB register. We
will use a macro instead of hardcoding this.

Fixes: b97e9b94 ("rsi: Add new host interface operations")
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>