USB: ehci: omap: fix kernel panic with rmmod

Sets the regulator values to NULL if they are not defined. This
is required to fix the kernel panic in exit path when EHCI module
is removed on the platforms where EHCI regulator are not set.

Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>