staging: wlan-ng: replace macro with inline function in prism2mgmt.c

checkpatch gives the following message for the p80211rate_to_p2bit
macro:

CHECK: Macro argument reuse 'n' - possible side-effects?

To fix the message, replace the macro with an equivalent inline
function.

Signed-off-by: Tim Collier <osdevtc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>