usb: musb: core: honour initial transceiver state

As the usb transceiver driver usually starts first, it should already
have default_a variable set according to ID pin state, so don't
override it. In case default_a was not changed by trasceiver, it will
default to 0 and this code will work as before.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>