USB: musb: add Blackfin driver to MUSB framework (v2)

- replace MUSB_FIFOSIZE register to MUSB_TXCOUNT, cause no MUSB_FIFOSIZE
   register on Blackfin
- use #ifdef to replace #if defined()

Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>