usb: musb: fix build breakage

This patch fixes the compilation brekage which
commits 208466dc ("usb: otg:OMAP4430: Powerdown
the internal PHY when USB is disabled") and
fb91cde4 ("usb: musb: OMAP4430: Power down
the PHY during board init") introduced when
building a OMAP2-only kernel.

  LD      .tmp_vmlinux1
arch/arm/mach-omap2/built-in.o:(.data+0x7ce0): undefined reference to
+`omap4430_phy_init'
arch/arm/mach-omap2/built-in.o:(.data+0x7ce4): undefined reference to
+`omap4430_phy_exit'
arch/arm/mach-omap2/built-in.o:(.data+0x7ce8): undefined reference to
+`omap4430_phy_power'
arch/arm/mach-omap2/built-in.o:(.data+0x7cec): undefined reference to
+`omap4430_phy_set_clk'
arch/arm/mach-omap2/built-in.o:(.data+0x7cf0): undefined reference to
+`omap4430_phy_suspend'
make: *** [.tmp_vmlinux1] Error 1

Reported-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>