Since regulator_enable() is going to be marked as __must_check in the
next merge window, always check regulator_enable() return value and
print a warning if it fails.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>