staging/easycap: make OSS compilation optional instead of ALSA

OSS is deprecated yet currently it is reported to be more stable
therefore we keep it but make it optional
Revert the conditional compilation:
	add  CONFIG_EASYCAP_OSS and kill EASYCAP_NEEDS_ALSA
move oss-only code from easycap_sound.c to easycap_sound_oss.c

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