usb: musb: make Davinci *work* in mainline

Now that the musb build fixes for DaVinci got merged (RC3?), kick in
the other bits needed to get it finally *working* in mainline:

 - Use clk_enable()/clk_disable() ... the "always enable USB clocks"
   code this originally relied on has since been removed.

 - Initialize the USB device only after the relevant I2C GPIOs are
   available, so the host side can properly enable VBUS.

 - Tweak init sequencing to cope with mainline's relatively late init
   of the I2C system bus for power switches, transceivers, and so on.

Sanity tested on DM6664 EVM for host and peripheral modes; that system
won't boot with CONFIG_PM enabled, so OTG can't yet be tested.  Also
verified on OMAP3.

(Unrelated:  correct the MODULE_PARM_DESC spelling of musb_debug.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Felipe Balbi <me@felipebalbi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>