usb: gadget: udc: renesas_usb3: should call devm_phy_get() before add udc

This patch fixes an issue that this driver cannot call phy_init()
if a gadget driver is alreadly loaded because usb_add_gadget_udc()
might call renesas_usb3_start() via .udc_start.
This patch also revises the typo (s/an optional/optional/).

Fixes: 279d4bc6 ("usb: gadget: udc: renesas_usb3: add support for generic phy")
Cc: <stable@vger.kernel.org> # v4.15+
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>