wifi: rtl8xxxu: use module_usb_driver

We can use the module_usb_driver macro instead of open-coding the driver's
init and exit functions. This is simpler and saves some lines of code.
Other realtek wireless drivers use module_usb_driver as well.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230307195718.168021-1-martin@kaiser.cx