rtlwifi: fix non-kerneldoc comment in usb.c

Fix spurious warning message when building with W=1:

  CC [M]  drivers/net/wireless/realtek/rtlwifi/usb.o
drivers/net/wireless/realtek/rtlwifi/usb.c:243: warning: Cannot understand  * on line 243 - I thought it was a doc line
drivers/net/wireless/realtek/rtlwifi/usb.c:760: warning: Cannot understand  * on line 760 - I thought it was a doc line
drivers/net/wireless/realtek/rtlwifi/usb.c:790: warning: Cannot understand  * on line 790 - I thought it was a doc line

Clean up the comment format.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>