staging: easycap: make functions local to easycap_main.c static

1. remove declarations from the header file
2. rearange code in main.c to reduce number of forward declarations

Cc: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>