drivers: sensor: lis2de12: fix read accel via spi

The lis2de12 sensor driver spi interface was calling spi read api.
This leads to a single byte operation on reading acceleration data
which is a multi byte operation. Fix it by adding a call to spi read
incr api instead. Added a length check to handle both single and multi
byte read properly.

Signed-off-by: Mayank Narang <narang.may77@gmail.com>