brcmsmac: Reduce number of entries in tx DMA rings

Currently up to 256 frames can be queued for each DMA ring. This is
excessive, and now that we have better flow control we can get by with
less. Experimentation has shown 64 to work well.

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