ARM: pxa: fix pxa3xx-u2d crash when ULPI not used

In case the pxa3xx-u2d driver isn't used, probing of ohci-pxa27x will cause an
ugly kernel crash (NULL pointer dereference in pxa3xx_u2d_start_hc(), because
struct u2d is NULL and clk_enable() call will crash the kernel, trying to access
it).

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>