phy: stm32: use NULL instead of zero

phy: stm32: use NULL instead of zero

devm_clk_get() and devm_reset_control_get() expect a const char *id for
the last arg, but a value of zero was provided. This results in below
sparse warning:

drivers/phy/st/phy-stm32-usbphyc.c:330:42: warning: Using plain integer as NULL pointer
drivers/phy/st/phy-stm32-usbphyc.c:343:52: warning: Using plain integer as NULL pointer

Instead of zero, use NULL

Reviewed-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20200708132809.265967-6-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>