staging: rtl8712: add spaces around '+'

Added spaces around '+' to improve readability and adhere to linux
kernel coding style.
Reported by checkpatch

Signed-off-by: Mitali Borkar <mitaliborkar810@gmail.com>
Link: https://lore.kernel.org/r/YG7uLQLGmAh97xB1@kali


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