[#349] invert the value of long_accel_mps2 for driving in reverse gear

Description

The vehicle does not drive backwards as it requires positive acceleration value long_accel_mps2 for reverse gear. This can be observed after doing correct gear change by fixes done in !227 (merged) and !228 (merged). For issue #349 (closed) even after

Notes for Reviewer

• Closes #349 (closed) (along with !227 (merged) and !228 (merged).)
• When the joystick/mpc_controls wants to go in reverse they produce negative long_accel_mps2.
• on negative long_accel_mps2, lgsvl_interface changes gear to reverse if velocity is 0
• once gear state is in reverse for input negative long_accel_mps2, lgsvl_interface produces positivie long_accel_mps2 to generate throttle.

Checklist

• Documentation was updated