drivers: sensor: lsm9ds1: use consistent variable name data

In both lsm9ds1 and lsm9ds1_mag drivers, the device data structure is
generally referenced with the variable name data.
However, in the initialization functions, the variable was named
lsm9ds1 (or lsm9ds1_mag for the magnetometer), which breaks this
convention.

Rename these variables to data for consistency across the driver.

Signed-off-by: Miguel Gazquez <miguel.gazquez@bootlin.com>