staging: rtl8192u: ieee80211: Remove MSECS macro

Remove driver specific macro MSECS since all instances of this macro
have been replaced by direct calls to msecs_to_jiffies().

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>