staging/easycap: convert comparison to NULL into boolean

convert if (NULL != ptr) to if (ptr)
convert if (NULL == ptr) to if (!ptr)

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