regulator: core: request only valid gpio pins for regulator enable

Commit 65f73508 ("regulator: core: Add core support for GPIO controlled
enable lines") introduced enable gpio entry in regulator configuration
structure. Some drivers use '-1' as a placeholder for marking that such
gpio line is not available, because '0' is considered as a valid gpio
number. This patch fixes initialization of such drivers (like MAX8952
on UniversalC210 board), when '-1' is provided as enable gpio pin in the
regulator's platform data.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>