usb: phy-keystone: Remove redundant of_match_ptr helper

'keystone_usbphy_ids' is always compiled in. Hence the
helper macro is not needed.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>