usb: storage: realtek_cr: Simplify residue calculation in rts51x_bulk_transport()

Simplify the calculation of 'residue' in rts51x_bulk_transport() and
avoid unnecessarily reassigning 'residue' to itself.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20250828162623.4840-3-thorsten.blum@linux.dev


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>