usb: musb: remove blackfin port

The blackfin architecture is getting removed, so we can clean up
all the special cases in the musb driver.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Aaron Wu <aaron.wu@analog.com>
Acked-by: Bin Liu <b-liu@ti.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
[arnd: adding in fixups from Aaron and Stephen]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>