musb: host side diagnostics tweaks

Random host-side MUSB updates, mostly relating to better diagnostics:

 + Improve diagnostics on host side:
    - tx flush fifo:
       * Avoid hundreds of duplicate TX FIFONOTEMPTY messages
       * make "Can't flush TX fifo" a warning, and say which endpoint
    - giveback:
       * use correct status code
       * show completion function name not just URB pointer
    - Fix annoying "1 bytes" (should be "1 byte")

 + Be more consistent about failing init of unusable fifo_mode

It's not clear why that "can't flush TX fifo" message appears, though
it might relate to disconnection; I see it not infrequently

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>