rtl818x_pci: make RSSI code more readable

remove the if-else chains and use switch-case to make code more
readable and avoiding long lines that broke in several lines

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

diff --cc drivers/net/wireless/rtl818x/rtl8180/dev.c
Simple merge