staging/easycap: Make code re-entrant

In order to allow multiple EasyCAP dongles to operate simultaneously
without mutual interference all static variables have been eliminated
except for a persistent inventory of plugged-in dongles at module level.

Signed-off-by: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>