brcmsmac: Use correct descriptor count when calculating next rx descriptor

nextrxd() is calling txd(), which means that the tx descriptor count is
used to determine when to wrap for determining the next ring buffer
entry. This has worked so far since the driver has been using the same
number of rx and tx descriptors, but it's obviously going to be a
problem if different numbers of descriptors are used.

Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Tested-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>