USB: cdc.h: ncm: typo and style fixes

Some typos were in the initial commit, make the spelling
according to the spec.

Add some more comments.

Also change constant names according to the style of the rest
of the file

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>