iio: magn: bmc150: use flags argument of devm_gpiod_get

Since 39b2bbe3 (gpio: add flags argument to gpiod_get*() functions)
which appeared in v3.17-rc1, the gpiod_get* functions take an additional
parameter that allows to specify direction and initial value for output.

Simplify driver accordingly. Furthermore this is one caller less that
stops us making the flags argument to gpiod_get*() mandatory.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>