usb: omap: ohci: Missing driver unregister in module exit

The un-registration of OHCI driver was not done in the ohci_hcd_mod_exit
function. This was affecting rmmod command not to work for OMAP3
platforms.  The platform driver un-registration for OMAP3 platforms is
perfomed while removing the OHCI module from kernel.

Signed-off-by: Keshava Munegowda <keshava_mgowda@ti.com>
Signed-of-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>