usb: musb: setup TXCOUNT for Blackfin musb

The Blackfin implementation of musb has a TXCOUNT register that needs to
be programmed when transmitting data.

Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>