rt2x00: Fix race condition when using inderect registers

Indirect registers require multiple calls to the CSR
register in order to access the indirect registers.
This must be protected under a lock to prevent race
conditions which could cause invalid data to
be returned when reading from the indirect register or silent
failures when writing data to the indirect register.

USB drivers where already protected under a mutex,
so rename the mutex and make PCI drivers use the mutex
as well.
This now means that BBP and RF registers are no longer
accessible in interrupt context. That is not a bad
situation since the slow behavior of accessing
those registers means we don't _want_ to access them
in interrupt context either.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>