rt2x00: Place mutex around USB register access

There is a buffer, csr_cache which is used to hold copies of data being passed
to the USB stack which can get corrupted if multiple threads attempt to access
CSR registers simultaneously. There is also the possibility if multiple
threads try to access BBP or RF registers for the multiple USB operations
needed to get interleaved leading to incorrect results. This patch introduces
a mutex to prevent such simultaneous access. The interleaved access problem
may also affect the PCI devices but if so that will be handled in a follow-up
patch.

Signed-off-by: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>