ath9k: rename rxflushlock to pcu_lock

ath9k: rename rxflushlock to pcu_lock

The real way to lock RX is to contend on the PCU
and reset, this will be fixed in the next patch but for
now just do the renames so that the next patch which changes
the locking order is crystal clear.

This is part of a series that will help resolve the bug:

https://bugzilla.kernel.org/show_bug.cgi?id=14624

For more details about this issue refer to:

http://marc.info/?l=linux-wireless&m=128629803703756&w=2

Cc: stable@kernel.org
Cc: Ben Greear <greearb@candelatech.com>
Cc: Kyungwan Nam <kyungwan.nam@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>