check_pycodestyle: fix return code on exit

When `check_pycodestyle.py` script finished, it was not checking
correctly its return code, some errors were not catched during
this period. This patch fixes the mentioned error and fixes
all python codestyle errors introduced.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I8771cf4b85847ac1f11d0b28f33d1d363cd60fd7