staging: wlan-ng: replace WLAN_CTL_FRAMELEN with inline function in p80211hdr.h

checkpatch reports a "CHECK" diagnostic for WLAN_CTL_FRAMELEN as the
macro reuses its argument, leading to possible side-effects. Avoid
this by replacing the macro with an equivalent function, named
wlan_ctl_framelen (as recommended in the coding style). All references
to the macro also updated accordingly.

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