iio: accel: bmc150: use common definition for regmap conf

bmc150_i2c_regmap_conf is defined three times (in bmc150-accel-core.c,
bmc150-accel-i2c.c and and bmc150-accel-spi.c), although the
definition is the same.

Use one common definition for bmc150_i2c_regmap_conf in all
included files.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>