greybus: use PTR_ERR_OR_ZERO()

Coccinelle points out that the macro PTR_ERR_OR_ZERO() handles the
frequent case of converting a pointer into either error code (if its
value is an encoded error value) or 0 (otherwise).  Switch some code
in gb_power_supply_register() to use that macro.  I have verified
this is true of the kernel we're now working with (arche-6.0).

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>