fix aea3 spi config + minor cleanup

[sgiraz]

What's new

• Fix spi configuration for chipMA730 (aka aea3). Now it compliant with the datasheet
• minor cleanup in aea3 encoder readings (see also https://github.com/robotology/icub-firmware/pull/420)

Note

• Tested on lego-setup with AMC board and aea3

[sgiraz]

Hi @marcoaccame,

please wait to merge until we have a confirmation test by @simeonedussoni.

[simeonedussoni]

hi @marcoaccame the PR-modified FW has been tested and runs smoothly on the wrist, can be merged.