phy: usb: Fix misuse of IS_ENABLED

While IS_ENABLED() is perfectly fine for CONFIG_* symbols, it is not
for other symbols such as __BIG_ENDIAN that is provided directly by
the compiler.

Switch to use CONFIG_CPU_BIG_ENDIAN instead of __BIG_ENDIAN.

Signed-off-by: Chen Li <chenli@uniontech.com>
Reviewed-by: Al Cooper <alcooperx@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Fixes: 94583a41 ("phy: usb: Restructure in preparation for adding 7216 USB support")
Link: https://lore.kernel.org/r/87czuggpra.wl-chenli@uniontech.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>