USB: phy: fsl-usb: fix missing static keywords

A recent change enabling compile testing of a Freescale dual-role
controller indirectly enabled a USB PHY driver to be built. That driver
in turn is missing a bunch of static keywords which results in warnings
like:

drivers/usb/phy/phy-fsl-usb.c:105:5: error: no previous prototype for 'write_ulpi' [-Werror=missing-prototypes]
  105 | int write_ulpi(u8 addr, u8 data)
        |     ^~~~~~~~~~

which consequently breaks -Werror builds.

Add the missing static keywords.

Fixes: 0807c500a1a6 ("USB: add Freescale USB OTG Transceiver driver")
Cc: stable@vger.kernel.org	# 3.0
Reported-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/4f9f5ff9-8eaa-4bd5-9331-37119f78e13f@sirena.org.uk
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://patch.msgid.link/20260717154957.1853976-1-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>