all: make license type consistent between source files and COPYING file

all: make license type consistent between source files and COPYING file

The license type of COPYING file is "LGPL-2.1+", but it is declared
as "LGPL-2.1" in the preamble of source files.

This patch fixes license type inconsistency issue in each source file.

Signed-off-by: SZ Lin <sz.lin@moxa.com>
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>