usb: s5p-ehci: fix a NULL pointer deference

This patch fixes a NULL pointer deference. A NULL pointer
dereference happens since s5p_ehci->hcd field is not initialized
yet in probe function.

[jg1.han@samsung.com: edit commit message]
Signed-off-by: Yulgon Kim <yulgon.kim@samsung.com>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>